Whenever a bird flaps its wing forward it creates forces called 'lift' and 'thrust', and it is these forces which move the bird up and forward. Hummingbirds have the ability to rotate their wings backward, thus creating downward 'lift' and backward 'thrust'. By alternating flapping their wings forward and backward, the up and down forces and forward and back forces cancel each other out, thus enabling the hummingbird to hover  in one place.

 

This ability to hover is very helpful to the hummingbird because it can hover long enough to suck all the nectar it needs from a flower.