For more information:
/// This is MyMethod, and it does stuff :)
/// <param name="a">'A' is the first parameter, where you
/// should put Object number 1</param>
/// <param name="b">'B' is the second parameter, where you
/// should put Object number 2</param>
public void MyMethod(object a, object b)
Edit: If you actually want the 'Description' attribute for reflection or something, you can't use it on parameters. Attributes can only be used on classes, properties, fields, methods and constructors - not parameters. That would be like placing an attribute on a local variable - kinda silly.