A tile map is a group of tiles, small images that act like a puzzle pieces which can be put together to create a larger image eg. the level you are playing on. This is good because it will take less time to build the level as instead of having to load each texture individually from files, you only need to load one file with the tiles within. This means the game will run faster and be more efficient.
I used pickel to create my own tile map, then proceeded to design a level using them:
