Call parking is a type of hold. The difference between hold and park is that when you place a call on hold, the held call stays with your phone and you can only retrieve it from that phone. With call parking, the call goes to a "parking spot" (an open line) marked "Park 1, Park 2, etc.," where it can be picked up from any phone in the system.
For example, if you're on a call and need to go back to your office to get a document, you could park the call and then pick it up in your office. Or you could park a call and hand it off to another person ("Louisa, you have a call waiting on Park 3").
Call parking is provided with the phone service, but phone admins decide which phones have the parking spots visible.
Admins: To enable or manage this feature for your company or individual users, contact Support.