Knit One Below (k1b)
& Purl One Below (P1B) Stitches
These particular stitches are definitely something that are much easier to show on video, but the basic idea is that you knit/purl into the stitch below where you normally would (as the name would suggest!)
The challenge of this stitch is knowing how to find and recognise the stitch that you need to go through, and understand what and where the 'stitch below' is.
It's not a hugely common stitch, but it is vital for patterns such as the fisherman's rib stitch.
I hope you find this video useful!