Download mode is the mode which allows you to install custom ROMs, perform hard reset, clear caches and so on. Of course, to install custom ROMs, you will need to install clockworkmod recovery on Galaxy Note 10.1, otherwise, you will not be able to flash custom firmwares.
To put the Samsung Galaxy note 10.1 in download mode, hold down Power and Volume Down buttons together for about 15 seconds. Your tablet will reboot and you will be greeted with a warning screen. Do not fret and press the volume up button. This will put your tablet in download mode.