Real-time credit card processing is when a user enters their personal information along with their credit card information into the order form, and they get a response as to if their credit card was accepted or rejected right away. Most merchant account and software combinations allow for this but some do not, so make sure if you want to do this that yours has the ability.
The other option instead of real-time credit card processing is manual credit card processing. What this means is if a user enters an order at midnight then it would go into a queue, which you would then have to check in the morning. If the order looked legitimate you would then process the credit card. Doing the credit card processing manually just adds another step in the process and the more steps there are the more chances there are that a mistake will be made.