The Dot Product (a/k/a inner product or scalar product) is a mathematical formula to calculate a single scalar value from two matrices. The formula for a Dot Product looks like this:
$$ A(a1,a2,a3) * B(b1,b2,b3) = (a1b1)+(a2b2)+(a3b3) $$
In 3D graphics a Dot Product is used to calculate the inner product of two vectors. The value produced is between the two vectors, so taking the above:
$$ -AB <= cos Θ <= +AB $$
Fun Fact: The dot product of orthogonal vectors (vectors oriented at 90° to each other) is always 0.
The dot product of a vector with itself will yield the square of its length.
$$ |v| = sq.rt(v*v) $$
Used in this fashion, the Dot Product will calculate length.