A trigger point is a self-sustaining, hyperexcitable site located in a skeletal muscle or its associated fascia. This point in the muscle is painful when compressed, located in a tense band (taut band) and can lead to a characteristic referred pain and sensory, motor and autonomic reactions in the body.
Trigger points are small, palpable hardenings in a muscle that are very painful when pressed. A muscle with a trigger point in it is also usually painful when stretched. A trigger point consists of small muscle fibers that are tight and cramped together ('contraction knots'). This makes the supply of nutrients difficult and waste products cannot be easily removed. The bundle of muscle fibers in which the trigger point is located is contracted and can be felt as a tight band in the muscle. The muscle becomes shortened and has poor blood flow. In this condition, use of the muscle will lead to exhaustion and pain: it becomes weakened. Other muscles will take over the role of the weakened muscle. If this continues, they will become overloaded and will also develop trigger points.

Symptoms that can be caused by trigger points are:

- myofascial pain, localized in a certain area, characteristic of the muscle; Tingling or numbness may also occur in this area;

- change in muscle function such as muscle shortening (this causes movement restrictions and stiffness in the joints) and decrease in strength;

-vegetative symptoms such as increased sweating, shivering, feeling unwell, skin hypersensitivity, dizziness/light-headedness, blurred vision or cold hands;

-coordination disorders, such as deteriorating handwriting or difficulty making precise movements.

-Trigger points in neck and/or neck muscles can cause headaches.

also see this nice explanation on Youtube