Mental math refers to the practice of doing calculations in your head. It is often used as a way to calculate an estimate quickly through the use of math facts that have been committed to memory, such as multiplying, dividing and squaring big numbers. Another cool trick is calculating any weekday in the past or future, being able to rapidly say which weekday it was, or is going to be, with a randomized date in time.
Check out Arthur Benjamin, a well-known mathmagician. You'll be amazed by what a brain can do, yours too.