Heck, I spent years earlier in my career as a .NET dev and I also don’t understand their naming. I wasn’t totally clear on what .NET core is. It’s not the biggest problem but the way they name things is quite confusing.
You know when movie franchises go through a reboot? That's very similar what Microsoft is doing with .NET Core. The 'Core' part of the name is hinting at a redesign based on a new runtime. They could've called it .NET Framework 6, but that doesn't give you any hints about the rewrite. I appreciate it's still not the best name in the world, but hopefully the analogy helped.
To add some additional information, whilst there has been a redesign, work has been carried out to allow a certain level of backward compatibility. This work is referred to by the name .NET Standard. Can think of this as re-implementing a selection of commonly used libraries from older .NET versions on the new .NET platform to make porting easier. The most up to date .NET Standard version is version 2.0.
