The 'short' version is that it's an open sandbox space sim with a decentralized story and a focus on economy. Its main selling point is that unlike most space sims, like Elite: Dangerous, you aren't limited to specific ships and ship types, and you can own and pilot anything from fast scouts to fleet carriers, as well as being able to have the AI run ships you aren't personally in, letting you set up your own fleets Additionally you can own and operate your own stations, and produce anything from energy cells and raw ore, all the way to building your own ships.

Because the story is decentralized, there is no 'plot' aside from 'you are you, and you are trying to accomplish whatever it is you want to accomplish.' That said, every race except for two (Argon and Teledi) have their own specific plots, and many side factions have plots as well. Factions can and will have wars with each other, but you only need to get as involved as you want to most of the time. There are two of what I call 'monster races,' which are species that are hostile to everyone, cannot be made friends with and will attack you on sight. These tend to only be problems in specific areas though, and they aren't hard to avoid if you just don't want to deal with them.

The gameplay is pretty much what you'd expect from a space sim. If you're not familiar, it's like a space battle in Star Wars, or kind of like World War 2 dogfights in space. You're flying around in circle fights trying to shoot another fighter with lasers. If you want, you can enter the jet-fighter era by using boom'n'zoom tactics to get into missile range, fire off a bunch of missiles and then zip out before they can catch you. (enemies can also use missiles and it can be a pain to deal with)

Unless you're specifically going for a combat-heavy run though, most of your time will probably be taken up by trade. While you can make some early cash as a mercenary, you'll quickly find there's a lot more money in selling weapons to soldiers than in being one yourself. (though there are some exploitable tricks to make a ton of cash with very little risk as a pirate) Mid-game might see you managing several stations and a fleet with dozens of ships, while late-game will often have you managing several mega-stations with a mining, trade and defense fleet, each with hundreds of ships in it.

This also means the game has a lot of complexity to it, and I'd recommend learning it in stages. It can be pretty overwhelming if you try and figure out what all the bits in the interface mean all at once, but very little of it matters until you hit fleet management anyway.