When you are editing a buffer, you might type something by mistake. Emacs allows you to undo all changes you make to a buffer (but not more than 8000 characters). Each buffer in Emacs keeps a record of the changes made to it individually, so the undo command applies to the current buffer. There are two undo commands:
Undo one batch of changes (usually, one command’s worth).
The same as above, but this command might not be obvious to type on some keyboards so it might be better to use the above command.
See Undoing Changes in SXEmacs User’s Manual, for more information on undoing changes.