Transaction rollback is probably what he meant. If that is a familiar topic to you the relation to undo should make sense and his point should be clear. If that doesn't make sense, look up ACID, Transactions, and Migrations in the context of a database.

I had a different interpretation. It's not uncommon in commercial code to use a sort of copy-on-write behavior where every edit creates a new row and marks the other one as dead (for instance, filling in or blanking out a column to mark it defunct). You leave an audit trail behind.

Which, with a little bit of work, can be used to revert to earlier versions.