What is (run on .NET Framework) while creating a .NET Core web development? [duplicate]

If the "Run on .NET Framework" checkbox is selected, the generated project will create an ASP.NET Core 2.1 application that runs on .NET Framework rather than .NET Core.

It is worth mentioning that ASP.NET Core 2.1 is the last version that supports .NET Framework. Look at the .NET Core and .NET 5 Support Policy for details.

.NET Core does not include .NET Framework, they are different runtimes and frameworks but have a lot in common

In 2002, Microsoft released .NET Framework, a development platform for creating Windows apps. Today .NET Framework is at version 4.8 and is still supported by Microsoft.

In 2014, Microsoft began writing a cross-platform, open-source successor to .NET Framework. This new implementation of .NET was named .NET Core until it reached version 3.1. The next version after .NET Core 3.1 is .NET 5.0. Version number 4 was skipped to avoid confusion between this implementation of .NET and .NET Framework 4.8. The name "Core" was dropped to make clear that this is now the main implementation of .NET.

Source: https://docs.microsoft.com/en-us/dotnet/core/introduction#net-core-and-net-5