The Interval LoopStart is the node that starts a loop which increases a variable within a user-defined interval by a certain amount. This is very handy for nodes inside a loop that take a continuous parameter. The current value is accessible via the scope variable loop_value . At the end of the loop you need LoopEnd, which collects the results from all loop iterations. All nodes in between are executed as many times as you specify in the dialog of LoopStart.
The input ports are just passed through to the output ports. You can add an arbitrary number of port pairs by using the “…” menu.