I am looking at a Chevron pattern for a scarf and I was wondering what psso means.
The pattern reads, K2, *inc1 by kfb, k4, sl 1, k2tog, psso, k4, inc by kfb.

So what do I do for the psso?

It's a decrease it stands for Pass Slip Stitch Over. What you do,for that pattern is Slip1 stitch, Knit 2tog and then pass the slip stitch over the knit stitch. There is a video on it here in the decrease section if you ever need to refer to it. That's how I learned it.