Hi Ruriko - welcome to Namepros!
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!
Here are a couple of articles I wrote for beginners to domain parking, hopefully they can answer all your questions:
Domain parking companies - how to make a return and
domain parking explained