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.
Save position of point in register r (point-to-register).
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 location. You can jump to the same position using the same register as often as you want.
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.