One of the most basic and essential techniques in crochet is the crochet yarn over. A yarn over is simply the act of wrapping the yarn over the crochet hook to create a new loop. The yarn over in crochet is abbreviated as "yo" in crochet patterns and is used in all crochet stitches, including single crochet, double crochet, and treble crochet. It is a fundamental skill that every crocheter needs to master to crochet a wide range of patterns and designs.