If you're using .Net Core version of csproj, it's the difference between e.g. <TargetFramework>netcoreapp2.1</TargetFramework> and <TargetFrameworks>net47;netcoreapp2.0</TargetFrameworks>.

I don't know if there's a good way to migrate the csprojs automatically (maybe https://github.com/hvanbakel/CsprojToVs2017?), but it shouldn't be that hard manually (by creating a new project), assuming you have just a few projects to migrate.