In addition to the ordinary mark ring that belongs to each buffer, Emacs has a single global mark ring. It records a sequence of buffers in which you have recently set the mark, so you can go back to those buffers.
Setting the mark always makes an entry on the current buffer's mark ring. If you have switched buffers since the previous mark setting, the new mark position makes an entry on the global mark ring also. The result is that 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 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 and earlier buffers.
Killing means erasing text and copying it into the kill ring, from which it can be retrieved by yanking it. Some systems use the terms "cutting" and "pasting" for these operations.
The commonest way of moving or copying text within Emacs is to kill it and later yank elsewhere it in one or more places. This is very safe because Emacs remembers several recent kills, not just the last one. It is versatile, because the many commands for killing syntactic units can also be used for moving those units. But there are other ways of copying text for special purposes.
Emacs has only one kill ring for all buffers, so you can kill text in one buffer and yank it in another buffer.