Well domain parking is a term for registering a domain name which leads to no services such as website or emails. Main purpose of domain parking is to reserve it for future, or sometimes it is done by someone who is looking to resell it for profit in future.
Domain parking can be classified as monetized - advertisements are shown to visitors. Non-monetized may have "Under Construction" or a "Coming Soon" message put up on the domain by the registrar or reseller. This is a single-page website that people see when they type the domain name. Domain names can be parked before a web site is ready for launching.
Most basic explanation for domain parking is that when someone visits your parked domain, they get to see a single "dummy web page" which may show ads.
If the domain parking service is a paid service, well that is up to registrar.