If I GRANT SELECT (or UPDATE/INSERT/DELETE) permissions to a user, then they can use those permissions to view data in a table. If I REVOKE the permissions, it’s the same as if the user never had them. They would need to be GRANTed permissions again to see the data.
However, if I DENY them the ability to see data, then that’s different. They can’t see the data, but a subsequent GRANT will not allow them to see the data because the DENY will still be in effect.
It’s a more permanent change, and should be used when you need to be sure that someone cannot see data, not when you are looking to remove permissions. To undo a GRANT, use REVOKE.