Wizards of the Coast is doing something that they've never done before in its Magic: The Gathering collectible card game: They're putting new cards directly into the game's Modern format.
Typically, cards can only enter into Modern by first being playable in Standard (a format that uses only the most recent sets. With the newly-announced 249-card Modern Horizons, however, they'll be printing cards both old and new to make their modern debut. Two examples of new cards can be seen below:
As for the old cards, WotC says they'll be reprinting cards from pre-Modern sets that would otherwise only be playable in sets that allow cards from virtually all sets ever printed such as Vintage and Legacy.
Master Horizons is scheduled to release June 14 (June 13 for Magic Online) with a pre-release the weekend prior. On Magic Online, packs will sell for $6.99. In paper there is no MSRP (as is normal going forward Magic).