A cue mark is an indicator to the projectonist at the projection booth that a reel is ending, and must change over projectors to the one threaded with the the next reel needed to continute, or just end the projector showcasing the film.
This has been rendered mostly useless in an actual theater due to new technologies, such as DCPs or or the ability to splice all reels into one large platter.
Although they did end up on older home media releases due to the transfers usually not using the original negatives, which don't contain the cue marks.
These are usually in the form of a black dot, but there's also a couple others, like one with ridges, or a simple circle drawn on.