Node Description & Purpose:
The Subtract family of nodes allow you to subtract the colour/scalar/vector coming into the Input by the color/scalar/vector coming into Input 2. The subtraction is component-wise, for example with a colour :
Input.red - Input 2.red
Input.green - Input 2.green
Input.blue - Input 2.blue
Node Type: Function
All nodes in this family have just the one setting, which lets you choose a node for 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.