Delta time and understimate( Im not understanding the delta time)

Question

Hey you guys im with problems of understanding these problems with delta time. Its in Pong-3 of course CS50 - Introduction to Game devlopment. Im not understanding why dt is there. dt is a variable not mentioned in all code. BUt is callled in render process in language LUA. Its like a local variable but is not pre-defined or another way of introduce that variable. I will give the code and wait for... some answer.

    --[[
    GD50 2018
    Pong Remake

    pong-3
    "The Paddle Update"

    -- Main Program --

    Author: Colton Ogden
    cogden@cs50.harvard.edu

    Originally programmed by Atari in 1972. Features two
    paddles, controlled by players, with the goal of getting
    the ball past your opponent's edge. First to 10 points wins.

    This version is built to more closely resemble the NES than
    the original Pong machines or the Atari 2600 in terms of
    resolution, though in widescreen (16:9) so it looks nicer on 
    modern systems.
]]

-- push is a library that will allow us to draw our game at a virtual
-- resolution, instead of however large our window is; used to provide
-- a more retro aesthetic
--
-- https://github.com/Ulydev/push
push = require 'push'

WINDOW_WIDTH = 1280
WINDOW_HEIGHT = 720

VIRTUAL_WIDTH = 432
VIRTUAL_HEIGHT = 243

-- speed at which we will move our paddle; multiplied by dt in update
PADDLE_SPEED = 200

--[[
    Runs when the game first starts up, only once; used to initialize the game.
]]
function love.load()
    love.graphics.setDefaultFilter('nearest', 'nearest')

    -- more "retro-looking" font object we can use for any text
    smallFont = love.graphics.newFont('font.ttf', 8)

    -- larger font for drawing the score on the screen
    scoreFont = love.graphics.newFont('font.ttf', 32)

    -- set LÖVE2D's active font to the smallFont obect
    love.graphics.setFont(smallFont)

    -- initialize window with virtual resolution
    push:setupScreen(VIRTUAL_WIDTH, VIRTUAL_HEIGHT, WINDOW_WIDTH, WINDOW_HEIGHT, {
        fullscreen = false,
        resizable = false,
        vsync = true
    })

    -- initialize score variables, used for rendering on the screen and keeping
    -- track of the winner
    player1Score = 0
    player2Score = 0

    -- paddle positions on the Y axis (they can only move up or down)
    player1Y = 30
    player2Y = VIRTUAL_HEIGHT - 50
end

--[[
    Runs every frame, with "dt" passed in, our delta in seconds 
    since the last frame, which LÖVE2D supplies us.
]]
function love.update(dt)
    -- player 1 movement
    if love.keyboard.isDown('w') then
        -- add negative paddle speed to current Y scaled by deltaTime
        player1Y = player1Y + -PADDLE_SPEED * dt
    elseif love.keyboard.isDown('s') then
        -- add positive paddle speed to current Y scaled by deltaTime
        player1Y = player1Y + PADDLE_SPEED * dt
    end

    -- player 2 movement
    if love.keyboard.isDown('up') then
        -- add negative paddle speed to current Y scaled by deltaTime
        player2Y = player2Y + -PADDLE_SPEED * dt
    elseif love.keyboard.isDown('down') then
        -- add positive paddle speed to current Y scaled by deltaTime
        player2Y = player2Y + PADDLE_SPEED * dt
    end
end

--[[
    Keyboard handling, called by LÖVE2D each frame; 
    passes in the key we pressed so we can access.
]]
function love.keypressed(key)
    -- keys can be accessed by string name
    if key == 'escape' then
        -- function LÖVE gives us to terminate application
        love.event.quit()
    end
end

--[[
    Called after update by LÖVE2D, used to draw anything to the screen, 
    updated or otherwise.
]]
function love.draw()
    -- begin rendering at virtual resolution
    push:apply('start')

    -- clear the screen with a specific color; in this case, a color similar
    -- to some versions of the original Pong
    love.graphics.clear(40/255, 45/255, 52/255, 255/255)

    -- draw welcome text toward the top of the screen
    love.graphics.setFont(smallFont)
    love.graphics.printf('Hello Pong!', 0, 20, VIRTUAL_WIDTH, 'center')

    -- draw score on the left and right center of the screen
    -- need to switch font to draw before actually printing
    love.graphics.setFont(scoreFont)
    love.graphics.print(tostring(player1Score), VIRTUAL_WIDTH / 2 - 50, 
        VIRTUAL_HEIGHT / 3)
    love.graphics.print(tostring(player2Score), VIRTUAL_WIDTH / 2 + 30,
        VIRTUAL_HEIGHT / 3)

    -- render first paddle (left side), now using the players' Y variable
    love.graphics.rectangle('fill', 10, player1Y, 5, 20)

    -- render second paddle (right side)
    love.graphics.rectangle('fill', VIRTUAL_WIDTH - 10, player2Y, 5, 20)

    -- render ball (center)
    love.graphics.rectangle('fill', VIRTUAL_WIDTH / 2 - 2, VIRTUAL_HEIGHT / 2 - 2, 4, 4)

    -- end rendering at virtual resolution
    push:apply('end')
end


                          

Answer 1

The Love update docs mention delta time briefly.

dt (Delta Time) holds the time in seconds since the last time the update function was called. Under normal circumstances this should be a small fraction of a second. The Love framework calls the update function for you repeatedly as part of your game loop. It also defines and keeps track of the dt variable somewhere within the framework, which is why you don't see it defined or introduced anywhere. You only have access to it from within the update function (unless you pass it elsewhere).

Why would you need the delta time?
Because you can't guarantee that update will always get called at a consistent rate. Imagine you have a player character that you move 1 pixel every time update is called. If your game is running at 60fps then your character moves 60px in 1 second. But of your game drops down to 59, or 50, or 12fps, then your character will slow down too, which is not usually what you want.

You can use the delta time to keep a consistent speed for your character. Multiplying the desired speed per second (60px) by dt will get you the exact amount to move each frame regardless of how often update is getting called.

That's what's happening in your code in places like this:
player2Y = player2Y + PADDLE_SPEED * dt

PADDLE_SPEED is the amount to move per second, multiplied by the number of seconds since the last update. The result is added to the player's position.