To hem an item is to bring up the bottom edge of a fabric, folding up towards the wrongside of the fabric.   It's purpose is to finish this edge and to shorten it.  You always hem the bottom of a pair of pants, skirt, shorts, or sleeve, but you will find yourself doing hems for other projects as well.  This is why learning how to do a hem is staple in learning how to sew.  Professor Pincushion shows you the easy way in creating a hem for any project.