Orb slots are a feature of The Defect, which limits how many orbs you can have channeled at once. If you channel more orbs than you have orb slots, then the rightmost orb will be evoked.
Player can increases and decreases the number of Orb Slots by playing cards or under effects of relics. The Defect can have at most 10 Orb Slots or have zero Orb Slot.
Inserter - Gain 1 Orb Slot every two turns.
Runic Capacitor- Start each combat with 3 additional Orb Slots.
Frozen Core- Channel 1 Frost if there is empty slot.