can't understand the pattern

Cast on 42 sts.
Row 1: (RS) Purl
Row 2: K1. *(K1.P1.K1) in next st. P3tog. Rep from * to last st. K1.
Row 3: Purl
Row 4: K1. *P3tog. (K1.P1.K1)in next st. Rep from *to last st. K1.
Row 5: Purl

I don't understand how Row 2 and 4 work. What does it mean to (K1.P1.K1) in next st. P3tog?

YOUR ANSWER

What it means is that you make three stitches out of one.

You insert the right hand needle into the front of the next stitch, wrap the yarn around the point and pull through a knit stitch, but rather than slipping the old stitch off the left hand needle (as you normally would) you re-insert the right hand needle back into the original stitch from the back to the front and form a purl stitch.

Then re-insert the right hand needle into the same stitch again to form a knit stitch. Three stitches from one...