This dialog box appears if you attempt to open or create a client project on a Universal Naming Convention (UNC) path. By default, a UNC path is not a trusted location for a project. Your project might not run correctly when you attempt to debug or run from this location.
If you need to work with code inside Visual Studio that is located on a network or remote location (non-trusted project location), you can modify the .NET Code Access Security Policy using the .NET Framework Configuration Tool and the Code Access Security Policy Tool.
MSDN has an article on what’s required to add UNC or network locations as trusted project locations.