You can crochet by winding loops of yarn around a crochet hook, creating little knots that form patterns and — eventually — blankets, pillow covers, doilies, or hot pads.
If your brother wants to make an afghan out of rainbow-colored yarn, he should learn how to crochet, or to make textiles by looping yarn with a special hook. Crocheting is similar to knitting, but people who crochet use a different tool. The word crochet comes from French and is therefore pronounced "crow shay." The root word is croc, or "hook."