The purpose of the "using" keyword in C# is to import namespaces into your code. It allows you to access types and members from other namespaces without having to fully qualify them with their namespace names.

Namespaces are used to organize and group related classes, structures, interfaces, and other types within a program. They help avoid naming conflicts and make it easier to maintain and understand code. When you want to use a type or member from a different namespace, you need to either fully qualify it with its namespace name or use the "using" keyword to import that namespace.

Here's how the "using" keyword works:

1. Importing a Namespace:
When you use the "using" keyword followed by a namespace name, you are importing that namespace into your code. This allows you to use types and members from that namespace directly without specifying the namespace name every time.

For example, if you want to use the Console class from the System namespace, you can write:
```
using System;
```
Now, you can use the Console class directly in your code without mentioning the System namespace:
```
Console.WriteLine("Hello, World!");
```

2. Multiple Namespaces:
You can import multiple namespaces by using multiple "using" statements. Each namespace should be imported on a separate line.

For example, if you also want to use the Math class from the System namespace, you can write:
```
using System;
using System.Math;
```
Now, you can use both the Console and Math classes without fully qualifying them.

3. Scope of the "using" Directive:
The "using" directive has a limited scope within a file. It applies to the file in which it is defined and any nested scopes within that file. It does not affect other files or namespaces.

In conclusion, the "using" keyword in C# is used to import namespaces into your code, allowing you to use types and members from those namespaces without fully qualifying them. It helps organize and simplify your code by avoiding the need to repeat namespace names.