Simply domain parking is taking a domain that does not have a website running on it and letting a third party - called a parking company - place ads on it to which you share the revenue that comes in with the parking company. We do it to make money, keep domains indexed, or as something to do with them until they are developed into websites. Any income earned helps pay renewal fees of course!
First of all, Welcome. I would recommend Bodis for parking. I've parked at Sedo and GoDaddy and i prefer Bodis for there 100% share and landing page options.