It is just an intentionally simple cross platform library that offers setting up a window, handling input devices, rendering simple 2D graphics, and playing sounds. The appeal is that it is cross platform, and very easy to use.
The learning curve for the library is negligible compared to the learning curve of making a game from these basic building blocks. It is comparable to learning how to use a set of tools, like socket wrenches and screw drivers, compared to learning how to fix a car.
But I don't think it is something that would be commonly used for commercial games right now. More commonly, people will use a professional game engine, like Unity, or Unreal engine, which will be much more complicated to learn than SFML, but offers a lot more built in tools, like 3d graphics, VR, animation, physics, etc. They are sort of like IDE's for games.
Due to the sort of monopoly that Unity, and Unreal engine have on the game engine industry, it's a pretty safe bet that they'll be around for a long time. Although they will keep releasing new versions with more features. It would be similar to the situation with Microsoft Word, Windows, IOS, Photoshop, etc.
I still recommend to just start with SFML though. It's good practice for learning basic programming skills, and you can get started easily. Maybe start with something like pong.