"WHAT IS PRERELEASE?

Your very first chance to play with the cards from a brand-new set.

A  week before the set is released, you can head to your favorite local  WPN member game store to crack open some packs, build a deck, and try  out all the exciting new cards and mechanics in a friendly, informal  tournament.

WHAT DO I GET?

Prerelease events are your first  chance to play with a set, and Prerelease packs get you in on the game!  Find themed inserts, one random spindown counter, boosters, and any  promo cards included in these packs."

- https://magic.wizards.com/en/play-events/prerelease