You can pick up your box this weekend, or wait until next Friday, the 14th, when the set releases.
Modern Horizons is the first Magic set to skip Standard legality and aim straight for Modern. Inside are new-to-Modern cards, with a mix of new cards and reprints that will become Modern-legal for the first time.
The first 72 boxes ordered will come will come with a Buy a Box card. This special card is not included in the set, but is playable.