In battle, you can spend elixir to summon cards from your deck. Weaker cards cost less, and stronger ones cost more. You'll want to have the strongest cards in play that you can manage, but it's important to plan ahead. Always make sure you have elixir in reserve. You won't do any good by summoning a powerful creature if you're then left with no elixir to spare. Over time, you'll see your elixir gradually refill, but don't put yourself in a position where you have to wait so long for that to happen that you're taken out of play.