If you read the file that you are editing in the example above (/etc/default/grub), you will notice that the very first couple lines instruct you to run
update-grub after making changes in order to update the actual file that grub reads to "get its instructions" (/boot/grub/grub.cfg). Note that you must actually run it with the
sudocommand first as you need root privileges to actually run the command (which is why the poster above said to type
sudo update-grub). This will cause the changes you made to be written to /boot/grub/grub.cfg. The very next couple lines tell you that you can read the full documentation of options in that file (again, /etc/default/grub) by typing
info -f grub -n 'Simple configuration'.
That said, set GRUB_TIMEOUT to -1 if you want to set the "grub time" to be indefinite. In other words, it will never automatically boot. You will have to make a selection.
Finally, to answer your question, here are the descriptions of those "grub hidden lines" straight from the above-referenced documentation:
Wait this many seconds for a key to be pressed before displaying
the menu. If no key is pressed during that time, boot
immediately. Unset by default.
In conjunction with `GRUB_HIDDEN_TIMEOUT', set this to `true' to
suppress the verbose countdown while waiting for a key to be
pressed before displaying the menu. Unset by default.
I hope this helps!