When you decrease, you are removing stitches from your knitting making that area narrower.

K2TOG

The most common decrease is known as the k2tog (knit two stitches together).

The k2tog creates a decrease that slants to the right.

When you work a k2tog, you need the second stitch to stay on top, meaning it needs to remain visible when you knit it.

Let’s create a k2tog as follows: k1, k2tog.

How: Knit peg 1. Move the stitch from peg 2 to peg 3 (the k2tog takes place on these two pegs). Knit peg 3, be sure to lift the two loops off the peg (treat them as one loop).

Move the stitches inwards, and the decrease by 1 stitch has been created.

SSK

The ssk creates a decrease that slants to the left (the opposite of a k2tog).

The ssk, stands for slip, slip, knit, which translates to loom knitting as skip, skip, knit. However, there is a small twist in our looming. We can’t translate this directly, you see on needles the stitches sit a certain way and the slipping of them changes their position, while on the loom the stitches sit always front facing (be it if we are purling or knitting).

How does it need to look: When you create a ssk, you need the first stitch to stay on top, meaning it needs to remain visible when you knit it.

Let’s create a ssk as follows: k1, ssk.

How:
Knit peg 1.
Move the stitch from peg 3 to peg 2 (the ssk takes place on these two pegs).
Knit peg 2, be sure to lift the two loops off the peg (treat them as one loop).
Move the loop on peg 2 to peg 3.
Move the loop from peg 1 to peg 2, and the decrease by one stitch has been created.
Alternate method:

Knit peg 1
Lift loop from peg 3 and hold it with your fingers (or put it on the peg to the left).
Move the loop from peg 2 to peg 3
Get the loop you are holding and place it back on peg 3.
Knit peg 3. Be sure to lift the two loops off the peg (treat them as one loop). Move the stitches inwards and the decrease by one stitch has been created.

These two decreases are usually paired together. I like to knit a few stitches at each side before doing the decreases, this gives the edges a more finished look.