A hydrogen economy is a hypothetical future economy in which the primary energy used for automobiles and other vehicles as well as creating electricity comes from hydrogen and renewable energy sources, like windmills and solar panel.
The reason to use this is to replace the use of petroleum. It would also stop the use of fossil fuels which causes carbon dioxide along with many other greenhouse gases, such as carbon monoxide and methane, to be created. These float up into the atmosphere, damaging it. This is bad for the ecology.