Depending on what you mean by that there are two possible answers.
1) To split water into hydrogen you have to use a lot of energy to split the water (H2O) into the constituent parts. This normally involves using a lot of electricity to break the bonds that hold the hydrogen and oxygen together. More simply if you think about two strong magnets stuck together and how difficult it is to pull them apart this is the job of electricity.
2) Hydrogen is found in pairs (H2) so to split it requires a lot of energy. Hydrogen is best friends with itself and has one of the strongest bonds in chemistry. It isn’t until you get to very hot temperatures >2700 C that hydrogen starts to break down into atomic hydrogen