On Error Resume Next causes execution to continue with the statement immediately following the statement that caused the run-time error, or with the statement immediately following the most recent call out of the procedure containing the On Error Resume Next statement. This allows execution to continue despite a run-time error. You can then build the error-handling routine inline within the procedure.
On Error Resume Next Err.Raise 6 ' Raise an overflow error. MsgBox "Error # " & CStr(Err.Number) & " " & Err.Description Err.Clear ' Clear the error.