The figure-of-nine loop is a type of knot. It is tied as a figure-of-eight loop but with an extra turn before finishing the knot - hence its name. It may also be described as a stevedore's knot tied in the bight.
It can jam badly in any rope type, but is somewhat less likely to jam than a figure-of-eight loop.
It is sometimes used instead of a figure-of-eight loop to attach a rope to an anchor point or belay. It has both greater strength and bulk than a figure-of-eight loop, so is normally used only in thin rope.