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!)

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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.

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It's not a hugely common stitch, but it is vital for patterns such as the fisherman's rib stitch.