What does the exclamation mark mean in this JS code?

Question

I am taking course for JS and while I couldn't figure out how to keep track a function once called or not even though I googled it. So I looked at the code written by instructor and I tried to understand but I can't figure out what the exclamation mark does in the if statement.

 var started = false;
$(document).keydown(function(){
  if (!started) {
      $("#level-title").text("Level " + levelNumber);
      nextSequence();
      started = true;
  }
});