Wrap & turns are used when making short rows which add shaping to your work. When working a short row, you work partway through a row or round, do a wrap & turn, then work back to the other end of the row or round without finishing the first one. Most often you do several wrap & turns in an even number on both sides of your work.
Wrap & turns are slightly different when you're on a knitside or a purlside, because your yarn starts in a different place (in front or in back). There are two sections of videos below, one for knitside wrap & turns and one for purlside wrap & turns.
If you haven't already, familiarize yourself with the different styles of knitting before continuing. There are three different ways to hold your yarn when knitting and you want to make sure you're watching the right technique for you.
w&t - wrap & turn
KNITSIDE WRAP & TURNS
PURLSIDE WRAP & TURNS