Whenever a player casts a spell from their hand, that player exiles it, then exiles cards from the top of their library until they exile a card that shares a card type with it. That player may cast that card without paying its mana cost. Then they put all cards exiled with this enchantment on the bottom of their library in a random order.
If you cast a spell without paying its mana cost, you can't choose to cast it for any alternative costs, such as overload costs. You can pay additional costs, such as kicker costs. If the spell has any mandatory additional costs, you must pay those.