Parked domains are domains that are not currently being used by the domain owner, but rather are leased to another company that profits from advertisements and links placed on the parked domain.
You may have come across a domain that has a lot of links on the main page, but no real web site to browse; this is a parked domain.
All domains are usually parked by the registrar by default. For example, if you were to buy a domain from GoDaddy.com, you would immediately see links and ads placed on your site by GoDaddy, until you purchase a web hosting account and change the nameservers to your new web hosting account. You can also intentionally park your domain for a very small profit each month.