In order to maintain ownership of a domain, you must renew it yearly. If you don’t, it “expires”. An expired domain goes through several stages before being released again for general registration. The process differs slightly from one registrar to another but it’s more or less the same. GoDaddy explains what happens to domains that expire with them here.
There is always a learning curve in buying domains with the purpose of reselling them. Don’t hesitate to ask a lot of questions to those who went before you, participate in forums such as namepros.com and dnforum.com, keep abreast of industry trends via resources like domaining.com, and reach out to the Afternic and GoDaddy Aftermarket support teams.
There’s an important distinction to make here between domain flipping and website flipping. The latter mainly refers to buying and selling full websites. By full websites, I mean websites that actually have content and more often than not have traffic and revenue. When buying and selling websites, the domain name matters less as the main value there is the content, traffic, revenue, history/reputation, sustainability and growth opportunities. The same can’t be said for domain names where you’re just selling “the name” and hence it’s all that really matters.