Node Description & Purpose: The Add family of nodes allow you to add the colour/scalar/vector coming into the Input by the color/scalar/vector coming into Input 2.
With Add Vector the addition is component-wise:
Main input.X + Input 2.X
Main input.Y + Input 2.Y
Main input.Z + Input 2.Z
Node Type: Function
Settings: 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.