Rehearsal marks are used by ensemble musicians to refer to a natural starting point for rehearsing some section of the score. A rehearsal mark looks like this:

You might also find rehearsal marks useful for your own purposes:

To specify the start and end of practice loops.

As a temporary bookmark when you are working on a section of the score. You can then quickly return to it using the Go To Rehearsal Mark command (shortcut "qgr") in the View menu.