Just to further what Jim said the “reset” (or on newer trailers “DSI fault”) light will come on while the system is purging and trying to light. It will attempt 3 times to light. If it cannot light after 3 tries it will lockout which means the red reset light will then stay lit. To reset you turn the switch off, wait 5 seconds and turn it back on. (The manual doesn’t specifically mention the operation of the light but it does as described above)
By the way, some of us have added LED’s or a lighted switch that will stay on while the switch is on as a visual indicator the hot water system is operating. I believe the switch stays lit on newer trailers where the switch and fault light are integrated into the KIB monitoring panel.
The product link in my post above is bad. A switch like this should work: