Sk2po (slip 1, knit 2 together, pass slipped stitch over) is a double decrease, decreasing your total number of stitches by two.

How to work a Sk2po decrease

So I have cast on 11 stitches and worked a few rows in stocking stitch, on the next knit row the instructions are :

K4, Sk2po, knit to end.  (9 sts)

I have knit the first stitch of the row and am now ready to work Sk2po:

Step 1

Step 1 – Slip the next stitch from the left hand needle to the right hand needle purlwise.

Step 2

Step 2 – Knit the next 2 stitches together.

 Step 3

Step 3 – Pass the slipped stitch over the stitch formed by knitting 2 stitches together.

A double descrease completed.

On the above sample I have worked a few more rows with a Sk2po decrease in the middle of each knit row. You can see how this decrease causes the left and right edges of the fabric to slant evenly in towards the middle.