Surplus is the excesssssss of something. In economics specifically the overproduction of a good. So like when Bieber Fever ended there was a surplus of posters. Then, normally goods are shipped to other places, like overstock.com or go overseas.
Commonly Chinese made goods are cheaper. This isn't just because they are made with cheaper labor, but they are overproduced. For instance, the violin I wanted to buy was only 35 dollars. It's that cheap because, they were overproduced in china, so they are selling them overseas.
In the U.S there are stores that sell government surplus. They will sell sleeping bags, uniforms, K-Rations, and other military items like that.