If an error occurs when your program is running, BASIC diagnoses the error
  and  displays  a message telling the nature and severity of the error, and
  the program line and module where the error occurred.   Your  program  can
  handle  most  errors  with  WHEN ERROR constructs.  A WHEN ERROR construct
  contains a block of code called a protected region that  is  monitored  by
  the  compiler for the occurrence of an error.  If an error occurs within a
  protected region, control can be transferred  to  either  an  attached  or
  detached  handler.   If you do not include an error handler, BASIC handles
  the error.  The severity of an error determines whether or not the program
  aborts when the error occurs.