A call can be placed into “Park” by pressing any park button which has a green light. Once a call is place in park, that park slot will show a red light. To retrieve the call from park, press the park button again. You cannot send a call to a park slot which is already red. If you accidentally try, don’t worry the call is not lost, it just remains active on your phone.
Parking a call is often the best way to get a call to another user. Once a call is parked, you can ring another extension to notify them that a call is waiting for them on Park 1, or Park 2, etc. They can then pick up the call at their convenience, rather than having to immediately answer a call which was directly transferred.