It is the opposite of an after-party. Also known as pregaming.
Phil: Yo! What are we doing tonight?
Jeremy: lets go to that party.
Phil: Wait, lets throw a before party
Jeremy: YEAH DUDE
*they high five*
basically, a pre-party except it suggests there is also an after hours. Useful for those eating doses or beans for the main event, since they take up to an hour to take effect, and may last longer than the main event.
Before party is at nine, doors open at ten, headliner is at midnight, what are you doing for after hours?