Edit: it is a rainmeter or raingauge, pluviometer, as the dictionary says.
You can see how many rain has fallen in a certain period of time. You can read in the meter there was 22 mm of rain fallen. That is per m2, in this case over 3 days. With this method you can register how much rain is fallen in a month, year, during summer etc. My grandpa used to write down the number every day, so at the end of the year he knew how much rain felt. My FIL is a little farmer and is very interested in the whether. Every time he calls us he asks how many rain we had.... Although he lives sky wide just a few kilometres from us, there a hugh differents in the mm that has felt. Some days it rains overhere and they have had no rain at all. So, overhere in the Netherlands, you can find rainmeters all over the place.