A Lyot stop is a particular type of stop, i.e., optical element designed to limit which light rays pass through an optical instrument such as a telescope or camera. The aperture of the telescope functions as a stop, and more stops (typically like a wall with a circular opening/window through which light passes) surround the path of the light through the instrument.
There are a number of functions of stops, including serving as a baffle to prevent unwanted electromagnetic radiation from reaching the sensors. A Lyot stop is one type.
(Another type of stop, found in photographers cameras allows adjustment of the stop's aperture, providing the potential for better focus if the object of a picture is sufficiently bright and also controlling the issues of a very bright source.)