I don't have Windows, but I'm a software engineer...

When someone says a piece of software will no longer be supported, it almost always means that the software is no longer being developed or maintained, which in turn means the software is not guaranteed to work anymore because the developers are no longer fixing bugs and glitches. In very rare cases, the software is still being developed despite being unsupported, but despite development, it still isn’t guaranteed to work anymore.

So in the case of Adobe Flash Player, there's a 99% chance this means Adobe will stop fixing bugs and will stop making improvements. So, the software may run slower, you may encounter glitches/bugs, or it flat out may not work anymore, and no one will fix those problems. However, just because it isn't guaranteed to work doesn't mean it won't work. In fact, it probably will work for a while. I doubt it will suddenly stop working on Dec 2020. Though if a new version of Windows comes out, then I would start to wonder if it'd actually work anymore.

Hope that makes sense.