It’s a very small thing, a piece of metal you add to your sink valve to reduce water consumption. It is called a tap-nozzle if it does also reduce the pressure or aerator if it is simply an aerator. It costs about 4 euros for the entry price which is absolutely correct qua efficiency and goes up to 30 euros.
The goal is to use less water and more air. For that, this small aerator adds air in the water flow, as its name indicates it. It however gives the impression you have as much water as before while realising a water reduction around 50% (12L/min to 6L/min) but sometimes going up to 70%.
This small piece of metal does not need a special maintenance neither is subject to fix more limestone than normal. It is very easy to install and is paid back after a few weeks.
The majority of newly installed taps have already incorporated the aerator but you should check to be sure, it could make a difference on your invoice as well as for the environment.
Video to install the faucet aerator: