Hey Chris
Nobody is dump, when he asks something ^^ There are no stupid questions, only stupid answers.
Write the openhabian Image to your SD Card of the RPI. As far as I know, the boot “Things” Need to be on the SD Card. After the basic boot (driver loading …) the RPI can Access the USB Device and boot the OS from it.
The main Problem with SD Cards is, that the get damaged sectors from many writes. With this solution you place the IO intensive parts to the USB device and your SD is safe. And if it dies, your data is at the USB device.
Detailed information can be found here. Tom Dietrich can answer you every question to his baby ^^
regards
Michael