The Start Interval Node sets an Interval, which occurs every XSeconds (or Timeout (sec)). When the Start Interval Node fires, the Logic connected to the Output Pulse Socket will be repeated every X seconds. At the same time, OnStart is triggered immediately. An Instance ID will be created, thus serving as an input for the Reset Interval Node. If Reset Interval -- connected by the Instance ID -- is triggered, OnReset fires.
Important to note is that an Interval will continue firing until a Reset Interval Node is triggered. Without resetting it, it will run forever.