The colon, :, is the near look command. It is used to look at the square you are currently standing on. Unlike the far look command, you will get an accurate description of any dungeon features, traps, and objects on the square. Using this command takes no in-game time, unless you are blind, in which case it counts as a turn.
This command is mainly useful for viewing information about the current square when there are a lot of objects or you have forgotten what is on the square, and you don't want to take turns moving off and back on.
If you are blind, you will determine what objects are on the square by feeling around on the ground, the same as when you move onto a square containing objects. This can cause YASD if there happens to be a cockatrice corpse on the square and you are not wearing gloves, so be careful.