Stray light is light that reaches a telescope's sensors (or photographic plates) other than that intended. Light pollution can be thought of as a source. Unintended external light might have a path to reach sensors, or it could be light that the telescope is designed to capture, but reaches the wrong point, leaking to sensors. The design of a telescope and its instruments requires attention to avoiding the problem.
Stray light mitigation is efforts to minimize the problem. In some cases, this involves designing in extra structure simply to block the unwanted light (baffles).