Arialdo Martini has a very informative post about the Emacs mark ring and how it works. On the surface, it’s pretty simple. The mark ring is a ring buffer that records positions inside a buffer—or several buffers in the case of the global mark ring—and allows you to return to or otherwise use those positions.
When you dig a little deeper, you find that there’s a lot more to it. For example, the mark helps define a region in an Emacs buffer and that fact can be leveraged to perform all sorts of useful tasks.
The mark ring actually stores markers to a given position so they are automatically adjusted if the absolute position of a place changes. Thus, you can, for example, push a position onto the mark ring, add text above that position, and still jump to where the mark was set even though its absolute position has moved.
In addition to allowing you to push positions onto the mark ring, Emacs will also record positions for you. For example, if you jump to a new position, your old position is pushed onto the mark ring. That allows you to do things like search for a symbol and then jump back to where you started. The exact rules for when Emacs pushes the mark onto the mark ring or global mark ring are a bit complex. This Irreal post discusses a video the explains some of those complexities.
The simple mark ring mechanism enables all sorts of useful functionality. Take a look at Martini’s post for some examples.