The knit 2 together, or K2tog, is probably the first decrease every new knitter learns. It's fairly easy to do and creates a lovely right leaning decrease which is often used to mirror the SSK, a left leaning decrease.
When creating symmetrical shaping in garments, you will see the K2tog being used with the SSK in the raglans of sweater, the toes/heels of socks, or the crown of a hat. It's a tidy little decrease that is commonly used in most knitting projects.
The K2tog also comes in handy when knitters inevitably find they have too many stitches. Instead of worrying too much about where that extra stitch came from, a simple K2tog solves the problem without anyone noticing.
Insert the needle into two stitches
It's that simple. Take the right hand needle and insert it into the first two stitches on the left hand needle as if to knit. Wrap the working yarn around the right needle counterclockwise and pull the new stitch through. This will decrease 2 stitches to 1!