How to "comment-out" (add comment) in a batch/cmd?

The rem command is indeed for comments. It doesn't inherently update anyone after running the script. Some script authors might use it that way instead of echo, though, because by default the batch interpreter will print out each command before it's processed. Since rem commands don't do anything, it's safe to print them without side effects. To avoid printing a command, prefix it with @, or, to apply that setting throughout the program, run @echo off. (It's echo off to avoid printing further commands; the @ is to avoid printing that command prior to the echo setting taking effect.)

So, in your batch file, you might use this:

@echo off
REM To skip the following Python commands, put "REM" before them:
python foo.py
python bar.py

Use :: or REM

::   commenttttttttttt
REM  commenttttttttttt

BUT (as people noted):

• if they are not in the beginning of line, then add & character:
  your commands here & :: commenttttttttttt
• Inside nested parts (IF/ELSE, FOR loops, etc...) :: should be followed with normal line, otherwise it gives error (use REM there).
• :: may also fail within setlocal ENABLEDELAYEDEXPANSION