Accessing the x number of characters in a string (MIPS)

Question

I have this code:

.text
.globl __start
__start:li $v0,8
    li $a1,20
    la $a0,str
    syscall
print_str:li $v0,4
    syscall
print_endl:la $a0,Endl
syscall
    la $a0,str
    sb $zero,5($a0) # <--------- #
print_5chars: syscall

Exit:li $v0,10
    syscall
.data
    Endl:.asciiz "\n"
    str:.asciiz "____________________"

As shown to me,this code first gets a string input, then prints the string entered and after that it prints the first 5 characters of the string.My question is what does exactly the line i marked do?