From the Help file:

<quote>
The Me keyword behaves like an implicitly declared variable. It is
automatically available to every procedure in a class module. When a class
can have more than one instance, Me provides a way to refer to the specific
instance of the class where the code is executing. Using Me is particularly
useful for passing information about the currently executing instance of a
class to a procedure in another module. For example, suppose you have the
following procedure in a module:

Sub ChangeFormColor(FormName As Form)
FormName.BackColor = RGB(Rnd * 256, Rnd * 256, Rnd * 256)
End Sub

You can call this procedure and pass the current instance of the Form class
as an argument using the following statement:

ChangeFormColor Me
</quote>

In other words, in a form's module, it's a way of referring to the current
form. You could have used:

Forms!NameOfForm!PageSum = Forms!NameOfForm!RunSum - x
x = Forms!NameOfForm!RunSum

but if you'd had two copies of NameOfForm open, you'd have run into
problems.