@juergen_hubert #Copernicus Climate Data Store (CDS) has a short overview on what is ERA5, if you haven't read it yet: https://cds.climate.copernicus.eu/datasets/reanalysis-era5-single-levels?tab=overview
Though you'll probably want to understand the "reanalysis" part a bit better. For that, #ECMWF has a nice overview here: https://www.ecmwf.int/en/about/media-centre/focus/2023/fact-sheet-reanalysis
For jargon explanations in German, you could refer to #DWD's Weather glossary: https://www.dwd.de/lexikon.html