It's the return value to the C runtime environment specifying success or failure. 0 means success and any non-zero value means failure in some implementation-dependent way.
Is return 0 tells the pointer to reach to end of a program.
The program ends regardless, since falling off the end of a function causes it to return. But if you return nothing prior to C99, what you're really doing is the equivalent of this:
Since rv is uninitialized, the value is unpredictable and trying to read it will invoke undefined behavior. The C runtime environment could very likely access the return value from main, which could wreak havoc because the value is indeterminate.
Is there any other alternative in which is reduced in characters than return 0.
No. Why are you so hell bent on reducing characters in such a trivial place?
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...