Here is a simple answer. This just tells the BIOS which device to look at for an operating system. In older computers, when they still came standard with floppy drives, usually had the floppy as the 1st device, then the hard drive. When CD/DVD drives became popular, computer were usually set with the CD/DVD drive 1st, then the hard disk. When installing an OS from CD/DVD, you'll want it marked as the 1st device.
In your case, the 500Gb doesn't have an operating system on it, so your SSD should be first. When you set your 500Gb drive 1st, the system can't find an operating system on it, so it goes to the next device, your SSD.
Newer motherboards provide a boot menu at start up which will over ride the boot order settings.