Prerelease Parties are casual, open play events with no pairings and no round structure, designed to enable the maximum use of Prerelease Packs.
The idea is, with a Prerelease Party on the calendar, you never have to turn anyone away. So long as you've got product, you've got something to offer.
Note that Prerelease Party attendance will not count towards future Prerelease allocations and should only be scheduled in addition to your traditional, ranked Prerelease events. See more information here.