Backdrops are a way to work with complex node systems that let you associate a selected set of nodes by colour. You can add as many backdrops to your node structure as needed. The idea is to visually organize the structure so you can quickly identify the particular set of nodes.
You can customize the backdrops with different colours. For example, the upper part of a character's body could be in different shades of red, while the lower body is in shades of green. When you create a backdrop to the Node view, the default colour is red.
Once you have created a backdrop with nodes inside, you can move the backdrop to the desired location and arrange multiple backdrops neatly. You can also resize backdrops to accommodate the number of nodes in your rig.
You can select the nodes you want to add to the backdrop before creating it and vice versa—create an empty backdrop and then drag the desired nodes into the backdrop.
For your convenience, you can add backdrops as templates to the library—see About Templates.