It is the water level indicator, which shows the level of water in the tank through out the day, provided there should be uninterrupted electricity. When the tank is filled up to 1/4" level the lower most LED glows up. Similarly when the tank gets filled up to half level i.e., approximately half of the tank's water storage capacity, the 2nd LED from bottom at 1/2 level lights up. Similarly 3/4 and full LED will glow along with the continuous alarm sound when when tank is full. It is the water level indicator, with four level indication and alarm at tank full to prevent overflow of water from the tank.