I have occasionally had to take over the controls for new emacs users while pair programming because the undo, while logical and consistent, doesn’t work as it does in other software. Each consecutive sequence of undo commands, once interrupted by any command that is not an undo is considered itself to be an atomic undoable change. This means that there is no need for a redo command, just stop undoing and the last undo sequence can be undone. Because emacs has a large undo limit you can keep undoing even past sequences of undos back to the original edit points of the file’s history.

That probably made no sense if you haven’t already mastered it, but another way to think of it is that edits and undos produce a branching tree of all the history of changes being made to the file. The entire tree of changes can be navigated with one command (undo) because that command makes a traversal of the tree, kind of like a post-order tree traversal. This is sufficiently confusing in practice that seeing the nanigation through the tree via the popular undo-tree emacs package helps a lot. I use the undo-tree command when I want to get to a version of the file not on the current main line of edits.

[1] https://www.gnu.org/software/emacs/manual/html_node/emacs/Un...