You can recognize a subcomposition node by the icon in its top-right corner. The icon consists of two overlapping squares, symbolizing that the subcomposition node has a composition inside of it that you can view and edit.
You can open that composition by double-clicking on the subcomposition node on the canvas, or by right-clicking on the subcomposition node on the canvas or in the Node Library and choosing
. When you save changes made to the subcomposition, the changes apply everywhere the subcomposition is used — every instance of the subcomposition node in the composition you have open, and all other compositions that contain the subcomposition.