What causes a muscle knot and what is it exactly?

Could be... A muscle spasm, or it could be an injury or tear in the muscle. Either way, if you have significant weakness in a muscle, or loss of range of motion of a joint, can't walk, or can't reach or hold a object in your hand, you should see a doctor without delay. Imple muscle knots or spasms are frequently related to too little or too much stretching of the muscles. Yoga or tai chi may abe helpful.
Another thought. A "knot" in a muscle could also be a herniated trigger point. This is a relatively knew concept from the fascial distortion model of osteopathic medicine. Essentially, a piece of muscle from a deeper layer pokes through a hole in another layer, causing a knot, pain and limited movement. The treatment: simply push the knot back through the hole, with proper training of course.