Let’s picture such a hydrogen atom covalently bound to a larger molecule. We’ll call the rest of the molecule "R". It’s a big molecule, and it has a lot of hydrogens on it, but we’re only concerned with this particular hydrogen, which is bound to an atom hungry for electrons, an electrophilic atom. Oxygen is such an atom—it has a lot more protons in its nucleus than hydrogen, and it’s missing two electrons in its outer shell, so it loves to "share" electrons with hydrogen—much as the IRS loves to "share" the money you make at your job.