I have already told you about the .NET libraries right ? Base class libraries that come with .NET are organised into namespaces (And the same will apply to how you organise your software)
System, System.Data, System.IO, System.Drawing
The above are only a small sample of .NET namespaces. You will learn about the important namespaces as we go.
Before you can use anything from a library you will need to use the “using” keyword to include that thing in your software
using System;