Park domains are associated with your main domain such that any traffic to the parked domain is automatically diverted to the main domain. The domain that you have parked may not have an email or website of its own or may not even be active. You simply keep it reserved for future use. When you want to use it, you can redirect the traffic incident on the parked domain on the main website.
For instance, if you have a website called as Psychz.com, you can reserve domains such as "PPsychz.com" or "Psychz.net" so that the customers typing these addresses will also be redirected to your main website.