What do you do if you have some text you want to yank back, and then
you kill something else? C-y would yank the more recent kill. But
the previous text is not lost. You can get back to it using the M-y
command. After you have done C-y to get the most recent kill, typing
M-y replaces that yanked text with the previous kill. Typing M-y
again and again brings in earlier and earlier kills. When you have
reached the text you are looking for, you do not have to do anything to
keep it. Just go on with your editing, leaving the yanked text where
it is.
If you M-y enough times, you come back to the starting point (the most
recent kill)."