Just like it sounds, a knit two together is when you knit two stitches as if they're one. It's the simplest decrease there is!
A knit two together is a right leaning decrease. Its left-leaning companion is the slip, slip, knit. These two decreases are often used together to create directional decreases that mirror one another. Although if only one decrease is being used in a pattern, it's usually knit two together.
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.