As Enki said, it's just an app and a desktop program. But the problem is that the desktop version doesn't link very well to the mobile version. It's kind of annoying because unlike Session which synchronizes everything by default, you have to actually purposefully link your phone version to your desktop version to use it. I'm sure there's some security argument to be made there, but Session is still a pretty secure messenger and it doesn't require such a process.

It's one of quite a few reasons why I don't really use SimpleX all that much, the others being things like REALLY slow speeds, annoying message catch up (thus annoying notifications) if you're away longer than a day, and the lack of onion routing.