Kicker

Kicker is a static Keyword Ability that represents an optional additional cost. A spell has been kicked if its controller declared the intention to pay any or all of its kicker costs.

Expansion Info
Kicker was introduced into Magic Arena with Dominaria (closed beta).

Card/Color Distribution
Kicker can often be found on Creature, Instant and Sorcery cards in all five colors.

Video
Watch this explanatory video to see how this game concept works.