
Black Bolt is from a race of not-really-humans that have decided to call themselves the Inhumans, which is a bit non-descriptive. This essentially means that aliens took some humans long long ago and maximized their genetic potential, and now each member of the race is genetically evolved all the way down the line of their would-be descendants into something that isn't human anymore. It's a species made up of a bunch of subspecies, and no Inhuman looks quite the same as the next one. But of course all of the main players here still look human, just have strange powers, because you can't draw a comic about someone that doesn't look human. You just can't.
Black Bolt himself (while looking totally human) has the power to tear apart cities (countries? planets??? it depends on the source material!) with his voice. It means he's essentially mute in every day life, and just sort of murmurs under his breath if he wants to explode someone. This is regarded as understandably a bit of a dangerous power, so he was sequestered in his youth, and shortly after they finally decided it was probably safe to let him out, he accidentally got his parents killed. Oops. But they made him king of the Inhumans anyway, because that's how the Inhuman Genetic Council rolls.
So then he went on adventures, got married to (his cousin) Medusa, stopped his (very persistent) brother from stealing the throne about seven times, moved his kingdom to the moon, declared war on the US over stolen property, got kidnapped and replaced by an alien shapeshifter, got rescued, and declared war on an alien race. Then everyone thought he was dead for a while, which is where he is now: presumed dead.