1. Traffic either comes from links.
In this case we are talking of a domain of a former site that other people linked to and their links to that domain are still all over the internet.
2. Or traffic is type in. It is either generic words or phraser or typos of other domains. For example yaho.com. Where does the traffic come from? Yahoo.com advertises their site and some people mistype the domain.

Internet is real big. Recent case when Verisign pointed traffic of all unregistered domain names to their page it scored 20 million a day and that is only mistyped traffic of .com and .net domains.
Mistyped and unregistered traffic.

It is true that to make decent profits you need many traffic domains that are not easy to get. But when you work with it, you have a good chance collecting a good portfolio that will make money.