Text: In the case of text, as long as you don't navigate out of the text section you are working in, you can often still press COMMAND-Z (Mac) or CONTROL-Z (PC) on your keyboard to undo what you've just done, just as if you were in a word processing program. But navigate that cursor out, and it's saved as-is.
Deletes: In the case of deleting something you want back, that's where the "not great" comes in. It's deleted.
The first thing I had to learn with Weebly was to slow down and try to be very present and zen-like, so I did less un-undo-able things.
The other thing I learned was . . .
But, even if you mess up, rebuilding doesn't usually take much time.
To avoid absolute disaster, archive your site and use the files to restore the site as described in the "How do I archive my Weebly site?" post (see Archiving Sites, in the Categories listing, right-hand column.)