When a domain name that was previously registered expires, it goes through a grace period that allows the previous registrant one last chance to reclaim their domain before it becomes available to new buyers. If the previous registrant still has not renewed their domain after the 77 day deletion process, the domain name will officially expire and enter the Aftermarket.
The domain name industry is quite similar to the real estate industry in a lot of aspects. There are end users, brokers, consultants and domain flippers or “domainers”. Domain flipping works similarly to buying a house, renovating it (or even sometimes just sitting on it) and then selling it again at a higher price point. The gist of it is: you’re purchasing a domain name and betting it’s worth (or will be worth) more than you paid for it. If you’re right, you get a nice paycheck and move on. Those who make a living out of this just rinse, repeat, and scale.