When you work a Trellis Stitch you create the trellis-like effect by slipping stitches and then pulling those loops over a stitch in a later row. When worked in a pattern it creates a deceptively complex looking fabric although it's very simple to create!
It is used in the hat pattern Genip.
To start with you work slip stitches for 2 rows (or rounds) across 3 stitches. This give 2 long loops of yarn across the front of the work like this:
Now you knit to the middle of those stitches:
Next you take the tip of your right needle and lift both of the yarn loops onto the needle from below:
Now knit the next stitch:
Lift the yarn loops over the knit stitch to create your trellis stitch:
This is what the completed stitch looks like:
Why not try this technique out in the following pattern: