It means that .NET Framework 4.6.1 and all later versions will support .NET Standard 2.0. However, .NET Standard 1.5 & 1.6 will not work on .NET Framework 4.6.1

It means that .NET Framework 4.6.1 and all later versions will support .NET Standard 2.0. However, .NET Standard 1.5 & 1.6 will not work on .NET Framework 4.6.1. In other words, we removed the APIs from .NET Standard 2.0 that weren't implemented by .NET Framework 4.6.1.