Hi. Trust conception is CAS (code access security) for ASP.NET applications. From the point of view .NET framework all ASP.NET applications are applications runing locally, so normally it would run in full trust CAS level. However it's not always desirable. Since that ASP.NET trust levels were introduced. Setting ASP.NET trust level allows to grant CAS rights induvidually for any separate ASP.NET application.

You can read about it in more details in MSDN in this and related articles:
 http://msdn.microsoft.com/en-us/library/87x8e4d1.aspx