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 gives a thumb somewhere safe to rest.
Run your finger back along the edge until the sharpening stops. The flat, full-thickness area from there to the handle is the ricasso. On a folder, it is the small flat between the edge and the pivot, and it is where the tang stamp usually lives.
Its length varies a lot. Traditional bowies have a long ricasso you can wrap a whole finger around. Modern EDC folders often have almost none, so the edge starts right at the handle.
A ricasso is a safe surface. You can pinch it to choke up for detail work without touching the edge, which is how most chefs hold a kitchen knife.
It also affects sharpening. Where the edge meets the ricasso there is often a sharpening choil so a stone can reach the heel. Without one, the heel of the edge is hard to sharpen fully.