The features in this extension are included by the Web Tools in VS 2017 15.3 by default. You should remove this extension before upgrading to 15.3.
If you have upgraded to 15.3 without removing this extension, repairing Visual Studio should fix it. See https://github.com/aspnet/Razor/issues/1628 for more details.
This extension provides support for ASP.NET Core Razor Tag Helpers in Visual Studio 2017.
To learn more about ASP.NET Core and Razor Tag Helpers, see the Intoduction to Tag Helpers in ASP.NET Core on the doc.microsoft.com website. To create your own custom Tag Helpers, see Introduction to Authoring Tag Helpers in ASP.NET Core.