That is a drop checker.
It's used as a guide for co2 level in the tank.
Basically you add a liquid which is usually blue, it changes colour depending on how much co2 is in the tank water. The colour green is ideal, blue too little, yellow too much.
Co2 is added by adding co2 gas canister via regulator, bubble counter and atomiser. Not too complicated to do and the main aim of adding co2 is to help plants grow, good for plants such as carpet type grass plants.
The addition of more powerful lights and fertiliser is usually used in combination with co2.
Can create beautiful lush green planted aquariums but must be monitored and done carefully otherwise algae issues are likely to happen. Sounds simple but it's not really, lots of research is needed before embarking on a co2 project.