This decrease appears on the knit side of your work and is usually used on the left edge of the row. It also can be worked on the purl side of your work. It's often abbreviated as "k2tog" or "p2tog."
Normally, you would insert the RHN into the first stitch on your LHN. In this case, skip the first stitch completely, and put the RHN into the second stitch and at the same time into the first stitch on the LHN. Notice that the RHN is now through two stitches. Complete the knit stitch by wrapping the yarn under and over the RHN and carefully pulling it through both stitches. Allow the two stitches to slip off the LHN; let the new single stitch remain on the RHN. You have just decreased one stitch. To purl two stitches together, put the RHN into the first stitch on the LHN as if you were going to purl the stitch, and at the same time insert it into the next stitch on the needle. Complete the purl stitch as you normally would, carefully pulling the yarn through both stitches. Slip the two stitches off the LHN, and let the new single stitch remain on the RHN. You have just decreased one stitch. This increase appears on the knit side of your work.