What does :: (double colon) mean in DOS batch files?

:: is a label (inaccurately also known as comment label) can be, in practice, considered a comment just as REM is, because it is an “un-goto-able” label.

There are some differences between REM and ::, though. The main ones are:

• With ECHO ON a REM line is shown but not a line commented with ::

• A :: can execute a line end caret (that is, a ^ at the end of a line starting with :: makes the next line also a comment):

   :: This is a comment^
   echo but watch out because this line is a comment too
  

• Labels and :: have a special logic and can cause problems in parentheses blocks – take care when using them inside ( ). Example:

   for %%D in (hi) do (
       echo Before...
       :: My comment
       :: Some other comment
       echo After...
   )
  

  Outputs:

   Before ...
   The system cannot find the drive specified.
   After...