.NET Standard is versioned. Each new version adds more APIs. When a library is built against a certain version of .NET Standard, it can run on any .NET implementation that implements that version of .NET Standard (or higher).
Targeting a higher version of .NET Standard allows a library to use more APIs but means it can only be used on more recent versions of .NET. Targeting a lower version reduces the available APIs but means the library can run in more places.
[!INCLUDE net-standard-1.0]
[!INCLUDE net-standard-1.1]
[!INCLUDE net-standard-1.2]
[!INCLUDE net-standard-1.3]
[!INCLUDE net-standard-1.4]
[!INCLUDE net-standard-1.5]
[!INCLUDE net-standard-1.6]
[!INCLUDE net-standard-2.0]
[!INCLUDE net-standard-2.1]