ch(s) = chain(s)
Chains are used to start crochet foundation rows but also in turning chains to start new stitch work designs. They are one to more series of loops that have a bottom loop and two top loops that will rest on your hook when pulling through one. You can also pull through a chain but just using the front top loop only or the top back loop for a harder technique and different look to your project.
Watch the following crochet Lesson 1 for notes on starting crochet including: holding your yarn, tension, foundation chain, turning chain and double crochet.