What is the little red LED light on the Tank Monitor Panel

It indicates the DSI on the water heater has faulted. In other words the Direct Spark Ignition on the propane side of the water heater failed to light three times. The heater went into lockout and turned on the light. You can reset it by turning the switch off and back on.

If you haven't used the propane for a while it's usually a good idea to purge the lines by running the stove burners on high for about 30 seconds. Then once purged try to light the water heater again.