A tilemap, outside of the context of C2, is an image that you can devide in shapes of fixed size, in each shapes, you have a tile, which represent something (there may be a more precise definition).
Basically, it is fine for multiple reasons:
-potentially less memory being used (as the entire map is loaded in memory, rather than each tile separately)
-less file, and so less request to files (minor but I guess still nice to have.), and some tools helps you using them (like in RPG maker, where you can simply tile your world with a tilemap, or in C2 with the tilemap object).
In the context of C2, the tilemap object:
-helps placing tile of a given tilemap object easily in the layout view
-benefits from some optimisations to prevent it from being too heavy performance wise (for exemple, what is offscreen enough is not even checked to be rendered)
To import a tilemap in C2 (only tilemap based on rectangular shapes are supported), you need to create a tilemap object, set its texture to the tilemap you want, and then configure the size of each tile.
More infos should be here: