It's a space MMO sandbox game that's been in development for like 11 years now I think? It's still in the alpha stage. I think at one point it was the highest backed Kickstarter in history. Some people think it's fraud and they'll never deliver, others think they're working on it just slowly. I think at this point universally though, people are just starting to give up hope. The game is available and in a playable state, it's just nowhere near the promised state.

It also doesn't help that a lot of ships in star Citizen can be bought with real life cash for pretty exorbitant amounts. A cursory Google search (haven't played the game) tells me that the "600i Executive Edition" is the most expensive ship in the game, available for $25,000 real life USD.

I've heard the best comparison for the game is the presently finished game Elite: Dangerous. Star Citizen is basically saying they're E:D but better.