what is meant by pre-define error that may occur in vb application

The pre-defined exception classes are those which are defined in the .NET Framework itself.

They include common exceptions such as : FormatException, InvalidCastException, NullReferenceException and OutOfMemoryException.

However, it's possible to define your own 'custom' exceptions by inheriting (directly or indirectly) from the System.Exception class which is the base class for all exceptions in .NET.