Reveal the top X cards of your library. You may put any number of permanent cards with mana value X or less from among them onto the battlefield. Then put all cards revealed this way that weren't put onto the battlefield into your graveyard.
A permanent card is an artifact, creature, enchantment, land, or planeswalker card.
All of the permanents put onto the battlefield this way enter at the same time. If any have triggered abilities that trigger on something else entering the battlefield, they'll see each other.
If a card in a player's library has {X} in its mana cost, X is considered to be 0.
If a card in your library has no mana symbols in its upper right corner (because it's a land card, for example), its mana value is 0. Such cards can always be put onto the battlefield with Genesis Wave.
If you have fewer than X cards in your library, you reveal all of them.
You don't have to put permanent cards revealed this way onto the battlefield if you choose not to, regardless of their mana values.