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. The specific aim of a Lyot stop is removing a diffracted ring of light from the occulted bright object (the Sun or a star) within a coronagraph, specifically the diffraction caused by the aperture (analogous to the rings of an Airy disk). Even with the occulter blocking direct light from the star itself, the light that the telescope diffracts can cover up the view of surrounding objects.
(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.)