It is a series of stiches which results in a ribbed pattern. The knit one purl two is a classic pattern and commonly used for sweaters, mittens, and just about anything else you can think of. I would think (although I have no statistcs on hand) that it is the most common knitting pattern used.

It’s actually three stitches done in a row. However, since people usually hate to purl as they find it more difficult than the knit stitch, I can’t imagine many people would like to knit a garment with that sequence over and over. The same effect would be generated by Knit 2, Purl 1 and making the other side of the garment the “front” side.

Whether that particular sequence would result in something like a seed pattern or a ribbed pattern depends on the number of stitches across that you’re working on.

It’s not a stitch, per say, it’s a series of three stitches. The first stitch is a knit stitch, then you purl the next stitch, then purl the next one after that. If the pattern says to repeat this, then you start over again with knitting one stitch. This would result in a stretchy ribbed pattern, similar (but not exactly the same) to what you find on the cuffs of knitted sweaters.