Expired domains are domains that have been registered by individuals, businesses, or organizations, but aren’t renewed after the contract ends, or are deliberately terminated. That means that they available for re-registration. There are many reasons why someone would choose to abandon a domain; one reason could be that the web project failed, or that a domain portfolio went into liquidization. Domains can also end up becoming free due to the owner receiving warnings for possible trademark infringement. Re-registering can also result in legal consequences.
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.