With the release last year of .NET 5, Microsoft switched its platform development away from the 20-year-old .NET Framework to the newer, cross-platform, open source .NET Core. The .NET Framework has moved into maintenance mode, while the new .NET completes its separation from Windows release cycles with a new cadence of annual releases.
In that new cadence, .NET 5 is what’s referred to as a current supported release, with 2021’s .NET 6 intended to be the first long-term support version of the new platform. That gives it three years of support, as opposed to .NET 5’s support which ends sometime early in 2022, three months after the .NET 6 release. You can think of current releases as pioneering new features for developers who produce regular updates, mainly for consumer applications. Long-term support fits better with enterprise product lifecycles and support models.