In a lot of game, the map is a table where each cell contains a tile. Some of them are solids, others not.
Example in Mario, each block is a tile :
![](http://www.saltgames.com/wp-content/uploads/2010/01/NES_Super_Mario_Bros.gif)
Mario is as wide as the tiles, so he can to be on 2 tiles at the time. So you have to check these 2 tiles under him to know if Mario can fall down.