when you add the dll reference to an application. It means
that you can use the classes in that namespace.
But if you don't mention the imports(vb)/using(c#) statement
wherever in your application you are referring to that class
you will have to mention the complete class name. 
If you use the imports(vb)/using(c#) statement you can
directly mention the class name to use it.

for example
'with imports
imports test 'test is the DLL and it has addtion class in it
.....
 class myclass
{
'to create an object of the addition class here is the stmt.
......
addition a = new addition()
.......
}


Without imports

class myclass
{
 test.addition a = new test.addition()
}