The drive defined in the bios at the top of the 'boot order' section is what the PC tries to boot off. Often if there is more than one drive on a different channel (for example you have both SATA and IDE drives) there is also a setting to define which channel is chosen as the first.
In cases where you are certain the boot order is correct, and you see the appropriate drive try, but fail to boot (as is the case for a DVD, you see the drive light winking), then there are possibilities such as (a) the disk in the DVD is not actually bootable (wrong disc type), (b) the drive is not defined to be bootable in the bios (applies to some USB drives), (c) the disc is damaged or scratched (d) the drive itself is actually faulty (e) the bios is corrupted and needs to be re-flashed. (f) the drive cable is not working fully.
Use your imagination to test these possibilities - borrow a known bootable DVD, put your DVD drive in another PC etc. PC hardware diagnostic are always a case of test by replacement.