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. SeatGeek is a marketplace that people can use to sell their tickets on the secondary market.
Secondary market ticket prices fluctuate far more than primary market ticket prices. Tickets often fall below face value, but if primary market tickets are underpriced or hard to get, secondary market tickets prices can cost more than face value.
The upside is you can almost always use the secondary market to make it to an event long after tickets from the box office have sold out!