Typing C-x r <SPC> (
point-to-register), followed by
a character r, saves both the position of point and the
current buffer in register r. The register retains this
information until you store something else in it.
The command C-x r j r switches to the buffer recorded in register r, and moves point to the recorded position. The contents of the register are not changed, so you can jump to the saved position any number of times.
If you use C-x r j to go to a saved position, but the buffer it was saved from has been killed, C-x r j tries to create the buffer again by visiting the same file. Of course, this works only for buffers that were visiting files.