What is a knot in a muscle?

This is a very important question.  if you are a massage therapist, or on your way to be one.  Yet I believe, despite all the schooling, continuing education, and training people go through to get this license, very few can answer?  Am I right?

What is a knot in a muscle?

What causes a knot in a muscle?

And how is a knot in the muscle formed?

How would a massage make a knot in a muscle go away?