A charging handle on an M16 and AR15 rifle, is a manual mechanism which when pulled and released, loads a round into the firing chamber when a magazine is loaded, and is used to lock the bolt carrier back ( along with the bolt catch ) when the weapon is not loaded with a magazine.  It is also used to clear the weapon if there is a possible malfunction or to remove a round from the chamber for unloading. 
On the M16 and AR15 rifle it is located in the rear section of the upper receiver, just above the buttstock and is shaped like a T, with a spring latch usually on the left side of the weapon.