The wikipedia link above is good. Here's my own brief take.
Humans have colonized Mars, and to a lesser extent other planets and moons. The Asteroid belt is being mined for resources. Earth and Mars are at political odds with each other, engaged in an uneasy, cold war-like standoff. The Belters yearn for independence.
What throws everything into chaos is the discovery of a strange "protomolecule" of unknown origin, that could be turned into a powerful weapon, or something else. No one fully understands it, and everyone wants to get their hands on it.
The story follows many threads (which all tie together), but centers on the crew of one ship who somehow seem to find themselves in the middle of most of the action.
It's a classic space opera. It's not really "hard" sci-fi, but it attempts to be a lot more grounded in reality than stuff like Star Wars or Star Trek. Lots of very well-rendered space battles, and interesting characters with plenty of depth. Also unlike Star Wars or Star Trek: it's very serious. There are moments of levity, but for the most part the tone of the show tracks the seriousness of the events taking place.
My suggestion is to watch the first four episodes, and then decide if you want to continue. After four episodes I was all-in.
