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.
A single object or device in the node network which generates or modifies data and may accept input data or create output data or both, depending on its function. Nodes usually have their own settings which control the data they create or how they modify data passing through them. Nodes are connected together in a network to perform work in a network-based user interface. In Terragen 2 nodes are connected together to describe a scene.