In a water electrolysis process, water is split into its components hydrogen and oxygen using electricity. In detail, the hydrogen moves to the negatively charged pole, while the oxygen moves to the positively charged pole.
If the electricity required for electrolysis comes from renewable sources (sun/wind, etc.), so-called renewable or "green" hydrogen is produced. Other possibilities for producing green hydrogen include the gasification and fermentation of biomass and the reformation of biogas. All the processes mentioned can be classified as CO2-neutral.