MBTiles is a file format for storing map tiles in a single file. It is, technically, a SQLite database. Latest format version as of February 2018 is 1.3.
Tile based web maps are made up of millions of tiles. Imagine loading all of those tiles; it would be inefficient and slow. That’s where MBTiles comes in. The MBTiles specification is an efficient format for storing millions of tiles in a single SQLite database.
SQLite is ideal for serving tiles on the web or displaying directly on mobile devices because it’s used on so many platforms. If you’ve worked with SQL databases before, SQLite should feel very familiar. With SQLite, each database is self-contained and represented as a single .sqlite file. There’s no external setup required. You can copy a .sqlite file from desktop to a mobile device and have all its rows, tables, and indexes ready to be used. It’s a portable, single-file solution for storing and serving web maps.
MBTiles takes advantage of utilities found in SQLite for solving problems like duplicate imagery. Maps that cover large areas of solid color like ocean or empty land can contain thousands of duplicate, redundant tiles.
For more
http://wiki.openstreetmap.org/wiki/MBTiles
https://www.mapbox.com/help/an-open-platform/#mbtiles-in-action
