I understand that this is extremely interesting, but I don't have the chemistry background to understand why. Can someone explain it to me like I'm a 5-year-old?
Hydrogen has just one proton, and therefore just one electron. But there's room for two electrons in the lowest orbital. This means that a hydrogen atom can accept an electron from another atom, forming an ionic bond, or it can share an electron with another atom, especially another hydrogen atom, forming a covalent bond.
The simple version is that hydrogen is highly reactive and forms bonds with other atoms, so you pretty much never get hydrogen atoms without bonding to other atoms, as in H2 (molecular hydrogen) or H2O or a gazillion other compounds.
Under high enough pressure H2 will become a liquid, useful in rockets and maybe (?) in fuel cells, but it's still a molecular liquid, the same way water is a molecular liquid.
But under really high pressure (maybe at the center of Jupiter) the molecular bond is broken, and the atoms of hydrogen are not bound to other atoms, and that makes it a metal because now the electrons are free to flow freely throughout the liquid, rather than each pair of electrons being bound to a pair of hydrogen atoms in the molecule.
What was theorized, and now perhaps demonstrated (I gather there's still controversy about whether this team actually did what they say) is that under higher pressure still, the atoms of this liquid metal hydrogen (think liquid mercury for an analogy) will freeze into a solid metal.
They speculate (and I'm skeptical) that when they release the pressure the solid hydrogen metal may remain solid. (Mercury does not remain solid at room temperature, why would hydrogen?)
It's fascinating because solid metal hydrogen has never existed before on Earth, and never before been demonstrated to actually be possible. It will be even more fascinating if I am wrong and their speculation proves true and the stuff remains solid at room temperature, though as Igor points out above, it seems as though producing the stuff in industrial quantities would be impossible.