It's an external liquid crystal thermometer. On mine, the highest bar that shows color is the temperature it gives you. This may or may not be anywhere near the actual temperature of the tank.
lol ppl make me laugh that they dont know wot it was the way 2 read them is and its the same for 90% of them is it will chnage colour depending on how warm yes obviously but when a certain temp is showing green then thats the actual temp if the colour is slighty blue below the the green is still the actual temp same is if it shows i slight brown above the actual temp is still the green tbh ppl slag these thermo's down but i have both 1 of these and a proper inside 1 and the both show exact temp so tbh they work just aswell as any other with very very slight inacuracy.