A chain can be used for all sorts of reasons in a pattern but most often it is used as the starting place for a crochet project. In this case it is known as your foundation chain. Here's how to make a crochet chain:
Make a slipknot on hook, *yarn over and draw through loop of slipknot; repeat from * drawing yarn through last loop formed.
You can also see how to chain by watching this video; the crochet chain technique is covered about 35 seconds into the video: