The C.Fn-10 Superimposed display function controls whether the AF points are illuminated in the viewfinder when autofocus is locked. Some people find the glowing red brackets distracting.
0: On. One or more focus points glow in red when focus locks.
1: Off. The focus points are not illuminated unless one is selected manually. Some find this option useful when the constant blinking of the AF point in the viewfinder becomes distracting.