The secondary market is where cards are bought and sold after the initial retail purchase — think eBay, COMC, MySlabs, card shows, Facebook groups, and your LCS's singles case. It's the aftermarket for cards, and it's where the real price discovery happens.
When a card is pulled from a pack, its primary market transaction is done. From that point forward, every sale happens on the secondary market, where prices are driven entirely by supply, demand, player performance, and condition.
The secondary market is where most serious buying and selling takes place. It's also where you'll find the best deals if you know what to look for — and the biggest markups if you don't. Learning to navigate the secondary market is one of the most valuable skills a collector can develop.