Candy.com and Cars.com are good examples of generic, yet descriptive, domains. Consumers interested in buying candy or looking for cars may intuitively type these names in to their web browser. On top of that, it is extremely easy to remember Candy.com.
Roku.com was an excellent domain acquisition. In a vacuum, Roku doesn't mean anything. The domain name, however, is short and easy to remember, and through branding, Roku has established itself as the household name for streaming movie and television programs over the Internet.
When you think about it, many major brand names did not have much of a meaning before they were created, such as Twitter, Netflix, Roomba, or the NFL. Yet over time these companies have become household names.