Nodes are FoldingText's internal data structure.
When FoldingText reads text it creates a node for each line. Each node is then classified based on its text content and the content of surrounding nodes.
For example when FoldingText sees:
# My heading
- item one
- item two
It creates this (simplified) node structure:
- node: "# My heading", type: "heading"
- node: "- item one", type: "unordered"
- node: "- item two", type: "unordered"
Each node is assigned a "type" and each node is positioned in a parent/child hierarchy in relation to the other nodes. In this case the first line is classified as a heading, and the other two lines are classified as unordered children of that heading.
Nodes have other properties including a unique "id", a list of associated "tags", and many others. But the important thing to remember about nodes is that they correspond 1 to 1 with lines in your document. And they are used to give each of those lines a location and type in FoldingText's internal node tree document structure.