If you have opted in to over-limit fees and you make a purchase that would cause you to exceed your credit limit, the card issuer will approve the purchase and charge you an over-limit fee. The fee can be charged only once per billing cycle and it can’t be more than either $25 or the amount by which you exceeded your limit, whichever is less. (For example, if you charged $10 over your limit, then you could only be charged $10. If you charged $100 over your limit, you could be charged as much as $25.)
If you don’t opt in, a transaction that would push you over your credit limit would be declined, and you would not be charged a fee.
In reality, very few card issuers impose over-limit fees anymore.
Even if you opt in, you can opt out at any time.