A trigger point is a small knot in your muscle that can cause pain where it originates or further away, in a spot that seems completely unconnected.
Trigger points are responsible for a majority of all pain syndromes, often referred to as a Myofascial Pain Syndrome.
This simply means that pain is triggered from the muscle and fascia. They are usually described as micro-cramps, but the science is half-baked and their nature is controversial.
Unfortunately, there are therapists who do not consider these points as being the trigger for pain.
Nevertheless, you can learn to use self-massage to treat these tiny, painful and persistent areas of tension yourself.