This stands for AE (auto-exposure) and AF (autofocus) lock. Often it’s useful to fix the exposure settings and focus point ahead of taking a picture and on most cameras you can do this by half-pressing the shutter button, holding it in position, then reframing the picture. By default, the AE-L/AF-L button does the same, locking both the exposure and the focus, but you can also configure this button to lock either the exposure or the focus, not both, so it’s more versatile than simply half-pressing the shutter button.