Whenever you cut text out of a buffer with a kill command in GNU Emacs, you can bring it back with a yank command. The text that is cut out of the buffer is put in the kill ring and the yank commands insert the appropriate contents of the kill ring back into a buffer (not necessarily the original buffer).
A simple C-y (yank) command inserts the first item from
the kill ring into the current buffer. If the C-y command is
followed immediately by M-y, the first element is replaced by
the second element. Successive M-y commands replace the second
element with the third, fourth, or fifth element, and so on. When the
last element in the kill ring is reached, it is replaced by the first
element and the cycle is repeated. (Thus the kill ring is called a
“ring” rather than just a “list”. However, the actual data structure
that holds the text is a list.
See Handling the Kill Ring, for the details of how the
list is handled as a ring.)