Kicker is an optional extra cost. When you cast a spell that has kicker, you may pay the additional mana shown — for example, Kicker {1} means paying {1} more as you cast it. If you pay, the spell is 'kicked,' and its other text may give you a bonus effect. If you do not pay, you still cast the spell normally with its base effect.
In play, kicker adds flexibility. You can cast the spell cheaply when mana is tight, or spend more later for a stronger result. That makes one card useful at different points in the game and forces opponents to consider both versions. It also rewards planning: holding back enough mana can turn a small play into a game-changing one.
