In addition to the ordinary mark ring that belongs to each buffer,
Emacs has a single global mark ring. Each time you set a mark,
this is recorded in the global mark ring in addition to the current
buffer’s own mark ring, if you have switched buffers since the
previous mark setting. Hence, the global mark ring records a sequence
of buffers that you have been in, and, for each buffer, a place where
you set the mark. The length of the global mark ring is controlled by
global-mark-ring-max, and is 16 by default.
The command C-x C-SPC (pop-global-mark) jumps to
the buffer and position of the latest entry in the global ring. It also
rotates the ring, so that successive uses of C-x C-SPC take
you to earlier buffers and mark positions.