The first line:

namespace first

is the header of a namespace declaration. This header is similar to Java's package declaration. Essentially the class SumProg belongs to a namespace called first.

The using directive:

using System;

means that the classes of the namespace System can be used in the subsequent code without qualification. So we can refer to the Write method of the System.Console class by Console.Write. The method Sleep belongs to the class Thread of the namespace System.Threading. Hence we can use:

using System.Threading;
...
Thread.Sleep(3000);