If you were playing hockey, you would not simply hop on the ice at game time and hope for the best. You would warm up beforehand so that as soon as your skates hit the ice, you're ready to go.
Yeast needs the same kind of warm up to properly ferment your beer! If you just toss your yeast out of the package into your fermenter, your yeast are going to be a little shocked by the sudden climate change. A yeast starter gives your yeast a chance to warm up before you pitch it in your beer.
What is a yeast starter? In some ways, it is like a small beer batch that wakes up your yeast so they reproduce and get ready to eat up your wort and produce alcohol. Since yeast can reproduce a lot quicker in beer with a lower gravity, you will use a low gravity wort for your starter.The gravity should ideally be around 40% of the starting gravity of your actual beer recipe. And if you're brewing a 5 gallon recipe, you should make a starer that is about 2 quarts by volume.