If a customer at a restaurant orders their usual, it means they're ordering the same thing they always get — nothing new, unexpected, or out of the ordinary.
How's this? The word usual describes things that happen regularly or follow a familiar pattern. If you take the same route to school every day, that’s your usual way there. Usual can also apply to habits. For example, if you check the mail at the same time every day, then you have a usual time for checking the mail. Anything that is typical, regularly occurring, or expected can be described as usual.