The Tunisian purl stitch (Tps) is the subject of some discussion, owing to the fact that it is called such simply because it looks like a knitted purl; not because it is any particular stitch in reverse.
However, I’ve ranted about this a little already.
When you work this stitch, the yarn is held to the front. The hook is inserted from left to right (for the lefties) or right to left (for the righties) behind the vertical bar formed in the stitch in the row below. The yarn is wrapped around the hook and pulled back through. The resulting loop is left on the hook and the next stitch is worked, and so on. The return pass is the same as for Tss.
Here is an Instagram video, showing the stitch in motion. Below that are photos showing step by step: