Hydrogen is the simplest element known to man. Each atom of hydrogen has only one proton. It is also the most plentiful gas in the universe. Stars are made primarily of hydrogen.
Hydrogen has the highest energy content of any common fuel by weight (about three times more than gasoline), but the lowest energy content by volume (about four times less than gasoline). It is the lightest element, and it is a gas at normal temperature and pressure.
Pure hydrogen is not found in nature, so energy has to be used to separate hydrogen from the other substances it is stored in. Currently 95% of hydrogen is produced from natural gas, a fossil fuel.
Hydrogen appears clean according to promoters, but it will require hundreds of billions of dollars in infrastructure, will be generated with fossil fuels for the foreseeable future, is less efficient than electricity, and is dangerous to store and transport.