This file, once unarchived, contains the bootloader, baseband, and the rest of the system.
You'll need to have fastboot in your path, and to have the device in fastboot mode before you can use it (either "adb reboot bootloader", or press-and-hold both volume-up and volume-down then press-and-hold power).
For convenience, a flash-all.sh script is provided as well, which restores the entire phone to its factory state.
If you're only flashing some of the images, I strongly recommend flashing things in the same order as flash-all.sh. Specifically, flash the bootloader before the radio, and reboot after flashing the bootloader or the radio.
Don't forget that after flashing back to a factory state, your bootloader is still unlocked. Don't forget to lock it back in order to secure your device ("fastboot oem lock").
Hopefully this'll be useful to people flashing custom AOSP builds, as it provides a clean supported way to return to factory state.