AE-Lock – “Automatic Exposure Lock”
The AE-Lock feature allows your camera to expose for a specific lighting condition by memorizing the correct automatic exposure in a certain part of the viewing frame, ignoring the rest of the light in the frame. Every camera establishes the AE-Lock section differently, so check your manual!
Usually this is done by aiming your camera, while in one of the auto modes, at a certain part of the scene, and then pressing the AE-Lock Button.
On many Canon cameras, this is the so-called “Star” button (small * above the button)
On Nikon Cameras the button is marked as [AE-L]