Thank you so much!!!
You're right that the timer is barely explained, I will be adding a section to the library where you can read about all the mechanics at some point.
Basically, the timer shows you the turn order: whoever's timer will hit 0 first goes next! Most card games do not have a timer like this, although Wildfrost is a notable exception (I highly recommend it).
For example, if it's Fer's turn and Dari has 5 timer but the enemy has 2, that means the enemy goes next (unless Fer plays a card that makes Dari faster by -3 timer or slows the enemy down +3 timer).
After your turn ends, your timer goes up by a number based on your speed. A slow character might gain +12 timer, a fast character might only gain +6 (that means the faster character in this example takes twice as many turns).