what are $PS1 - 4 in linux

PS1    The value of this parameter is expanded and used as the primary prompt string.  
PS2    The value of this parameter is expanded as with PS1 and used  as
       the secondary prompt string.  The default is ``> ''.
PS3    The value of this parameter is used as the prompt for the select
       command 
PS4    The value of this parameter is expanded  as  with  PS1  and  the
       value  is  printed  before  each command bash displays during an
       execution trace.  The first character of PS4 is replicated  mul-
       tiple  times, as necessary, to indicate multiple levels of indi-
       rection.  The default is ``+ ''.

@Zenonymous