Select a node and the inspector shows its History.
A revision is recorded each time a save changes that node's settings (moving it around does not count — nobody wants forty "moved 3px" entries). Click Restore to put the node back to a revision. The current state is recorded first, so a restore is one more revision you can restore from.
Only the node changes. Its connections, and every other node, stay as they are. The newest 20 revisions are kept per node.