Kredits are the resources that you use to play cards and operate units (move and/or attack). The cost of each card is displayed in the upper left corner of the card.
You use Kredits to play cards and operate units. At the start of your turn your Kredits are replenished and your max Kredits increased by one to max of 12.
Cards that give you more Kredits are called ramp cards. Ramp decks aim to build resources in the early turns to play big, powerful units and orders sooner.
Sometimes it is better to spend your Kredits operating your units on the battlefield rather than play cards from your hand.