The picture above was taken by Alexei Watson who states that the new stock recovery in Android L has a few new features. You can now Reboot to Bootloader, and Power down.

Ignore it unless you know what you're doing. Reboot to bootloader: This will take you back to the bootloader menu, where you can access recovery mode and other options.

What these two commands do is simple. the first one is basically telling your computer to tell your computer to reboot the Nexus 5 to the bootloader.