It's grounded in basic trigonometry, i.e. it calculates the angle `theta` between two entities/vectors, `a` and `b`. If `theta` is close to 180 degrees, cos(theta) is -1, and cosine similarity dictates these are opposite concepts, i.e. unrelated.
Unrelated would be a cosine of 0. Being opposites is a very strong relation.
Unrelated would be a cosine of 0. Being opposites is a very strong relation.