They mean opting out of overdraft protection. Banks typically allow you overdraft protection, where instead of denying your sale at point of sale, they’ll pay it, charge you a fee, and let the transaction go through.

If you opt out of overdraft protection, it should instead deny the sale, and you don’t get to buy whatever. At my bank, they do this and then charge an insufficient funds fee, which is the same as the overdraft fee. It’s bullshit.