What is a 'sub-class' mean

[bnathvbdotnet]

I was going thru a book and he wrote a class in a class and it looked it like this..

Public class Request
Public class parameter
private m_paramName as string
private m_Paramvalue as string

Public property ParamName as string
....
....
end property

End class

End class

What does it mean by subclass? I am seeing the subclass first time...any advantages?

thanks