The secondary ticket market is made up exclusively of tickets hoping to be resold after their original purchase from a venue's box office, which is also known as the primary ticket market.
Secondary market ticket prices fluctuate far more than primary market ticket prices. They can often fall below face value, too. On the flip side, if primary market tickets are underpriced or hard to get, secondary market tickets prices often cost more than face value.
The upside is you can almost always still buy your way into an event on the secondary market long after primary market tickets have sold out.