AF-assist Illuminator
A lamp or patterned light that projects onto the subject to give autofocus something to measure in darkness.
In depth
Autofocus needs contrast, and in near-darkness there may be none. A plain lamp helps a little; a projected red grid pattern helps far more, because it supplies artificial edges at a known orientation. Its range is short — a few metres — and it is intrusive, so it is often disabled for ceremonies and wildlife.
Examples in practice
A flash unit's red grid pattern lets the camera focus on a dancer in a room too dark for the eye to read a face.