At some point in your knitting career, you are going to drop a stitch. When this happens, a stitch falls off your needle and unravels for several rows beneath the row you are working on.
First of all, don't panic. Knit until you've reached the dropped stitch. Insert a crochet hook (if you don't have one available, the top of your needle will also work) into the loop of the dropped stitch.
Now grab the bar of the stitch right above it in the crook of your crochet hook. Pull the bar through the stitch. If your stitch has dropped down several rows, repeat until your stitch is on the same row as the rest of your knitting.