Different nodes in graph correspond to GPU computation via shaders.
It works like in post-process when you have done some effect having put result into surface of render target texture.
Then you make it be input for the next pass with other effect.
Same way in a graph when the primary nodes have been done the next nodes can be passed. And after this process repeats.
Output from the prev node is input for the next node. It will be textures for example.