What it says on the tin. Beholders and beholder kin are given a number of rays that they can use. The entry for the Beholder-kin: Watcher notes that each eye can use one of its powers every round, in its given arc. Does this mean that a beholder should be able to dump 10+ magic rays every round as long as it commits to a full-attack action? Or is it one ray per round, of its choice? The texts are silent on this. One answer seems to make the thing too easy, and the other answer seems to make it obscenely brutal.
"Each of a beholder's small eyes can produce a magical ray once per round as a free action. During a single round, a creature can aim only two eye rays (gauth) or three eye rays (beholder) at targets in any one 90-degree arc (up, forward, backward, left, right, or down). The remaining eyes must aim at targets in other arcs, or not at all. A beholder can tilt and pan its body each round to change which rays it can bring to bear in any given arc." (MM 25)
This is the main restriction. The beholder can use all ten of its eyes in one turn, but only if it has targets in various different arcs to choose from.
Suppose, for example, there are three PCs directly in front of it, and one on its left side. It can aim three rays at one of the targets in front, or divide the rays one each amongst the three in front (or however it chooses to divide them)... and the hapless sod by himself on the left will get hit by another three rays.
Also note that the Antimagic Cone is not compatible with other rays, and takes up the entire arc by itself.