If you want to understand the inner workings of a composition, port popovers are an extremely useful tool. You can visualize step by step how each node contributes to the end result.
Open the port popover for the Fire on Display Refresh node’s Refreshed at Time output port by clicking on the port. The small window that appears is the port popover. As the composition runs, the port popover shows the information flowing through the port in real time.