Saving a position records a place in a buffer so that you can move back there later. Moving to a saved position switches to that buffer and moves point to that place in it.

C-x r >SPC
< R Save position of point in register R (`point-to-register').

C-x r j R Jump to the position saved in register R (`jump-to-register').

To save the current position of point in a register, choose a name R and type C-x r >SPC
< R. The register R retains the position thus saved until you store something else in that register.

The command C-x r j R moves point to the position recorded in register R. The register is not affected; it continues to record the same position. 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.