How to use the bases in the designations of tolerances in the drawings?

Question

CAD/CAM

Good day, gentlemen engineers and not so much.
I, as a student, am engaged in the task assigned to me in the discipline "Details of mechatronic modules, robots and their design". The essence of the task is the development of a gearbox according to the given parameters. In the course of the work, the question arose:
how to use bases when designating the tolerances of shapes and locations?
I climbed to look at examples, since they are in the tutorial, and found this:

Actually, the main question:
what does the G base have to do with this tolerance for end runout (is that it?)?