Download mode allows you to download and flash files on your Galaxy Note 2. You will need to connect your device to the PC through an USB cable. Usually Odin is the preferred PC software for flashing files on the Galaxy Note 2, but not the only one.
The download mode allows you to flash kernels, ROMs, PIT files, bootloaders and more. While installing an official update will keep your warranty intact, custom ROMs and rooted ROMs will usually void it. But there are some ways to reverse it and it's pretty easy too.
You also must be very careful about the files you flash on your device, because flashing a wrong file can permanently brick your Galaxy Note 2. I recommend you to search our websites for the ROMs you want to install and follow the guides very carefully. We test everything and you can be sure that you won't encounter any problems.
Computer software like Odin also offer you some options when flashing the file, so you have to make sure that the options are right. If you flash the right file, but choose different options, you might brick your Galaxy Note 2.
Entering download mode is very simple. First you will need to turn off your device. Once it's off, you need to press this button combination: volume down, power button and home button. Keep them pressed until the download mode shows up. Once you're there, you will be requires to press volume up to enter the download mode or volume down to cancel and reboot the device.
For those will custom ROMs there's one more way of getting into download mode. It won't work on all ROMs, but you can try it. Keep the power button pressed for a few seconds and wait for a menu to pop up. There you will have a reboot button. Tap it and see if you get more options. If you see the download option, then you can directly reboot in download mode from there.