When opening an account what is the best way to explain to employees the difference between "or" co-owner and an account labeled "and" co-owner?

My explanation would be that neither means anything in particular; most banks have eliminated the use of conjunctions in account titles.

On the payee line of a check, "or" means either payee can sign while "and" means all payees must sign. For many years, a lot of banks thought the same rule applied to account titles. It does not. The number of signers required on an account should be clearly indicated in the contract, not the account title.