The Multiply family of nodes allow you to multiply the colour/scalar/vector coming into the Input by the color/scalar/vector coming into Input 2.
A scalar is a single number. 1, 200.45, -45, -0.2 are all examples of scalar values.
A vector is a set of three scalars, normally representing X, Y and Z coordinates. It also commonly represents rotation, where the values are pitch, heading and bank.