Here are a couple of articles on behavior trees in general. Although RAIN differs from these descriptions in various ways, then general approach and terminology are similar.
There are generally 3 types of nodes in RAIN:
Decision nodes (or logic nodes):
These are typically Parent or Container nodes. Their purpose is to hold some set of child nodes, and then perform some logic that determines how the child nodes are run, in what order, etc. Examples are Parallel, Sequencer, and Selector.
Action nodes (or leaf nodes):
These nodes don’t have children, but instead are used to actually do something. Examples are Move, Detect, Animate.
These are Decisions or Actions that you can write for yourself.
If you have questions on a specific node and what it does, go ahead and post those and I’ll answer them.