In Ranked matches, each player has a limited amount of time (60 seconds) to make their plays before Sky conspires and ends their turn. You can see a green timer bar on the top of your End Turn button (bottom right of the playing field), depleting from left to right. Think before you play, but be fast!

Also note that completing an action in your turn (casting a spell, summoning a Unit, attacking) will also add 5 seconds to your turn timer.