Thus, the text becomes a comment for maintenance, having no impact on the operation of the procedure. There is a little overhead in using such comments, as the command processor must parse the extra characters on the statement line, but it is not much. I think it is well worth the cost, adding significantly to the ability to document the procedures. Of course, I haven't gone through all of mine yet to add appropriate comments, though I know they need them. That's another issue.
Recursion means a function calls itself.
It is a very effective technique in a batch file. It is especially valuable in collecting data from DOS utilities for use in the running batch procedure. Nearly all of my procedures use it. However, there is an obvious need to keep the routine from calling itself over and over again. That's where the double colon comes in.