This certainly won't help you. It depends on where you found the term.
In your bios, you set up the devices you want to boot from in order. Second & subsequent options are the secondary boot order
In bootloader literature, in can mean your other configured boot options in the boot loader
In other places, It can mean anything you boot off except the primary hard disk.
Give it to Groucho Marx, and I am sure he could add a twist to it