Excursions - GNU Emacs Lisp Reference Manual

It is often useful to move point “temporarily” within a localized portion of the program, or to switch buffers temporarily. This is called an excursion, and it is done with the save-excursion special form. This construct initially remembers the identity of the current buffer, and its values of point and the mark, and restores them after the completion of the excursion.

The forms for saving and restoring the configuration of windows are described elsewhere (see Window Configurations, and see Frame Configurations). When only the identity of the current buffer needs to be saved and restored, it is preferable to use save-current-buffer instead.

— Special Form: save-excursion body…

The save-excursion special form saves the identity of the current buffer and the values of point and the mark in it, evaluates body, and finally restores the buffer and its saved values of point and the mark. All three saved values are restored even in case of an abnormal exit via throw or error (see Nonlocal Exits).
The save-excursion special form is the standard way to move point within one part of a program and avoid affecting the rest of the program. It is used more than 4000 times in the Lisp sources of Emacs.
save-excursion does not save the values of point and the mark for other buffers, so changes in other buffers remain in effect after save-excursion exits.
Likewise, save-excursion does not restore window-buffer correspondences altered by functions such as switch-to-buffer. One way to restore these correspondences, and the selected window, is to use save-window-excursion inside save-excursion (see Window Configurations).
The value returned by save-excursion is the result of the last form in body, or nil if no body forms were given.
          (save-excursion forms)
          ==
          (let ((old-buf (current-buffer))
                (old-pnt (point-marker))
                (old-mark (copy-marker (mark-marker))))
            (unwind-protect
                (progn forms)
              (set-buffer old-buf)
              (goto-char old-pnt)
              (set-marker (mark-marker) old-mark)))

Warning: Ordinary insertion of text adjacent to the saved point value relocates the saved value, just as it relocates all markers. More precisely, the saved value is a marker with insertion type nil. See Marker Insertion Types. Therefore, when the saved point value is restored, it normally comes before the inserted text.

Although save-excursion saves the location of the mark, it does not prevent functions which modify the buffer from setting deactivate-mark, and thus causing the deactivation of the mark after the command finishes. See The Mark.

30.3 Excursions