The cauda equina is a cluster of nerves in the lumbar region of the human spine that controls essentially everything south of it. It's responsible for the motor function of legs and related extremities, the pelvis, and the anus.
It is called the cauda equina, "horse's tail" in Latin, because that's kinda what it looks like: a bunch of nerves weaving their way down the tail-end of the spine, much like the hairs of a horse's tail.
When these nerves are damaged or have pressure put on them from, say, a herniated disc, then you have cauda equina syndrome and will require surgery. With cauda equina syndrome, there is a serious risk of a number of unpleasant things, such as paralysis of the legs and inability to control your bowels.