Zoomable User Interface
TouchDesigner uses a Zoomable User Interface or Zooming User Interface (ZUI) to navigate through a project's networks and to interact with Node Viewers. The middle mouse button (and scroll wheel) control the zoom level of the selected interface.
You can use the scroll wheel to enter Components as you zoom into them. Similarly, if you zoom out you will move up to the parent network. This and other user adjustable settings for mouse scroll wheel and network zooming interactions are found under Edit > Preferences > Network.