Kicker is a keyword ability.
It is a static ability that allows you to pay more mana into a spell to get an additional effect.
Multikicker is a variant on kicker, where rather than paying the addition cost one, you can pay it multiple times, for an increasing effect each time you pay.
Amy has an Into the Roil in her hand and wishes to cast it. When casting it, she may pay an additional and draw a card when Into the Roil resolves.
Alternatively, Arnold has an in his hand and wishes to cast it that. Similarly, he may pay an additional , but this time he can do it as many times as he wishes. For each time he pays it, Gnarlid Pack will enter the battlefield with a +1/+1 counter
- 702.32a. Kicker is a static ability
- 702.32b. The phrase "Kicker [cost 1] and/or [cost 2]" means the same thing as "Kicker [cost 1], kicker [cost 2]."
- 702.32c. Multikicker is a variant of the kicker ability. "Multikicker [cost]" means "You may pay an additional [cost] any number of times as you cast this spell." A multikicker cost is a kicker cost.
- 702.32d. If a spell's controller declares the intention to pay any of that spell's kicker costs, that spell has been "kicked." If a spell has two kicker costs or has multikicker, it may be kicked multiple times. See rule 601.2b.
- 702.32e. Objects with kicker or multikicker have additional abilities that specify what happens if they are kicked. These abilities are linked to the kicker or multikicker abilities printed on that object: they can refer only to those specific kicker or multikicker abilities.
- 702.32f. Objects with more than one kicker cost have abilities that each correspond to a specific kicker cost. They contain the phrases "if it was kicked with its [A] kicker" and "if it was kicked with its [B] kicker," where A and B are the first and second kicker costs listed on the card, respectively. Each of those abilities is linked to the appropriate kicker ability.
- 702.32g. If part of a spell's ability has its effect only if that spell was kicked, and that part of the ability includes any targets, the spell's controller chooses those targets only if that spell was kicked. Otherwise, the spell is cast as if it did not have those targets.