What does multi kick mean in magic the gathering?

Multikicker is an additional cost, and functionally it is just like the regular Kicker mechanic, but you may pay it multiple times. The exact cost and effect of a specific Multikicker is listed on the card itself, as it can differ from card to card.

So take 'Comet Storm' which costs XRR, and has a Multikicker of 1. You could have X as 2, and Multikick it twice, meaning you pay 2, RR, and another 2 as an additional cost. Comet Storm's multikicker says "Choose target creature or player, then choose another target creature or player for each time Comet Storm was kicked. Comet Storm then deals X damage to each of them." so you pick three targets in total, and Comet Storm deals 2 damage to each of them.