When your computer restarts, BIOS will attempt to boot to the first device in the boot order you specified. If the first device is not bootable, your computer will try to boot from the second device in the boot order, and so on.
Note: In Step 4, we set the first boot device to the CD-Rom Drive as an example. As you can see in the screenshot above, the computer is attempting to boot to the CD but is asking for a confirmation first. This only happens on some bootable CDs and will not show up when booting to Windows or other operating systems on a hard drive. Configuring the boot order to boot to a disc like a CD, DVD, or BD is the most common reason for making boot order changes so I wanted to include this screenshot as an example.