Undo reverses a recent action. If you delete a sentence by mistake, choosing Undo may bring it back, as if you had taken one step backward in your work.
Many apps let you undo more than one action, so you can step backward through several recent changes. Some actions cannot be undone, and each app has its own limits.
Undo is helpful, but it is not a replacement for saving important work or keeping backups. Before making a large change, check how the app lets you restore an earlier version.