To make use of this tool just tap the map on your exact position and see what's around you. If you want to scan the area in search of just one kind of pokemon you should open the full list and tick the kind you want to see. Like this you'll enlarge your field of vision and can discover the exact position of a pokemon more easily.
On the other hand, each of the pokemon that appear do so with a clock counting down over its head showing how much longer it will be in the area. Once it gets to zero it will disappear from the map – and with it your chance to capture it. This app also has a series of tools to help you become the best trainer you can possibly be.