Very often, when people "buy" a domain name, they are only just starting out on a new website. Since getting a domain is the first step of many in creating a website, most people don't have any website at the time they register the domain. As such, they can't point the domain name anywhere.
Domain name registrars, the companies through which you register a domain, work around this problem by allowing you to point your domain name to a dummy web page. When visitors type the address of a domain that has been parked, they will see this placeholder web page in their web browsers. The content placed on the dummy web page varies from registrar to registrar, and it's possible that some registrars place advertisements or links to themselves on your parked domain. (Note that you will not earn money from those advertisements. You will only earn from the advertisements you place yourself on your website.)
There are of course other situations which might lead you to want to park your domain. For example, if you were buying many domain names because you think you might want to reserve them for future use, or because you can't decide between the different permutations of names and want to take precautions against those names being "stolen", then it's likely that you will want to park those domains temporarily till you either decide what to do with those names or till you actually get a website ready for those domains.