What Are "Knots"?

What even are these "knots" I always hear about from massage therapists?
Taken from the NASM textbook as follow:
     "Pattern Overload. Muscular imbalances are highly prevalent in today's society and  are often caused by pattern overload. Pattern overload is consistently repeating the same pattern of motion, such as baseball pitching, long-distance running, cycling, which with time places abnormal stresses on the body."

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