Back-button Focus
A configuration that moves autofocus activation from the shutter release to a separate rear button.
In depth
Separating focus from release means the shutter never re-focuses at the moment of exposure, and it gives continuous and single-servo behaviour from one setting: hold the button to track, release it to lock. It is the single most useful custom configuration on most cameras and takes a few days of practice to become automatic.
Examples in practice
Tracking a subject with the thumb held down, then releasing it to lock focus while the subject pauses and the shutter fires repeatedly.