This article is from the Stand-up Comedy FAQ, by Steven Rosenthal and Steve Silberberg (firstname.lastname@example.org) with numerous contributions by others.
A. At some point while you're on stage, someone will shine a light in your
eyes. This is the owner's/manager's way of saying "Your time is over, wrap
it up." You then have about a minute to get off the stage. Generally, the
light is used to make sure a show runs smoothly and on time. However it can
also be used to tell you that your performance bites or that the person
with the light has a petty vendetta against you.