The ricasso is the flat, unsharpened section of blade between the end of the cutting edge and the handle or guard. It carries the maker's mark on most fixed blades, and it gives you a safe surface to pinch when you want to choke up on the blade.
Run a finger back along the edge until the sharpening stops. The flat, full-thickness area between there and the handle is the ricasso. On a folder it is the small flat between edge and pivot, and it is where the tang stamp usually lives.
Length varies a lot: traditional bowies have one you can wrap a finger around, modern EDC folders often have almost none.
It is the only flat, full-thickness, non-cutting part of the blade, which makes it both the natural place for a stamp and the natural place to pinch. Pinching the ricasso is how most chefs hold a kitchen knife.
It also gives the plunge line and the sharpening choil somewhere to sit.
Where the edge meets the ricasso there is often a relief notch so a stone can reach the heel. Without one, the heel of the edge is difficult to sharpen fully and tends to develop a step over years.
Take your stroke into the notch and stop; do not try to sharpen the ricasso itself.