A prerelease is a Sealed tournament, basically. For this one, you'll get 5 Theros booster packs and 1 "seeded" booster pack for a color of your choice and 1 alternate-art version of the foil rare from the intro box of that same color. So, for instance, if you choose Red, you get the 5 booster packs, 1 seeded booster containing only Red cards*, and 1 alternate-art Ember Swallower. The seeded boosters are pseudo-random, and if I recall from Ravnica block, there's something like 4 to 6 possible card combinations for each color's seeded booster.
You'll also get a Hero card based on your choice of color, which is a special card used for a gimmick that'll run over the Prerelease, Release Weekend, and Game Day tournaments. That's a bit more complicated to explain, but the gist of it is that it's only used in a special format, so you won't be playing with it at the tournament.
It'll require an entry fee, though. Ours is $25, which is slightly more expensive than just straight-up buying 6 packs, so if you expect to win nothing you'd want to be content with the fun of playing for a few hours, and getting access to the cards early. The Sealed format is similar to a draft, except you open all of your boosters yourself and construct your deck out of the contents (no actual drafting occurs, but I'm using the comparison because drafts are the most common limited format, IIRC).
*This could be wrong. Technically, it has been said to contain "lots of" cards of the appropriate color, and to be "appropriate to" the Hero's Path you choose.
EDIT: Mein gott! How can a penguin have honor and be able to enter the battlefield tapped and attacking in place of an unblocked creature that is returned to its owner's hand!?