The previous answer is also vague. The dot product (inner product) of two vectors returns a scalar, usually in the [-1, 1] range that represents how orthogonal two vectors are. A value of 0 means they are perpendicular, a value of 1 means they are parallel and facing the same direction, and a value of -1 means they are parallel but facing opposite directions. They can be used for a number of other things, such as finding the squared length of a vector, the projection of a vector onto another, etc.

It's another word for inner product, which is used in physics.