Domain parking refers to monetizing otherwise unused domain names via pay-per-click advertising.
There are numerous companies that exist soley to monetize “parked” domains in this manner. Most domain registrars “park” their customers’ idle and unused domains and monetize them, often keeping the revenues for themselves.
If you have domain names simply “parked” with your registrar, you may decide you want to monetize them yourself. If you do, you need to select a domain parking company and then configure your domains so that web traffic to them goes to the parked page provider’s system.