Modern is a constucted format you can only use cards that are released after odyssey with the newer but not the newest card layout. Reprints work.
You still have following rules to create your deck:
- Minimum 60 cards
- Side deck maximum 15 cards
- No banned cards
- No cards from Alpha – Odyssey, unless they have been reprinted after odyssey.
read more here:
https://magic.wizards.com/en/game-info/gameplay/formats/modern
https://mtg.gamepedia.com/Modern
What is the most powerful deck in Moden right now?
MTG blog 