A surplus is something extra or left over. If your tree produces more apples than you can eat, you can make applesauce with the surplus of apples.
The army surplus store sells old or out-of-date clothing and equipment that the military doesn't need. If you earn more money than you spend, then you have a surplus of cash. The word surplus has familiar parts: sur- is a shortened form of super, meaning "extra" or "additional," and plus just means "more."