Starting with Android 4.4, your device has checked to make sure everything is peachy keen when booting up. If a problem was detected you would get a little alert, but that was it. Android Nougat will do a lot more to prevent your device from running corrupted software.
Google explains on the Android Developer blog that Nougat still performs the check at boot, but now you’ll get more than just a warning. If corruption is found, Android will start in a limited-use mode or it won’t start at all. This will act as a safeguard to prevent viruses and malware from taking advantage of the corrupted software.
The downside is it could make custom software more difficult to create. One slip up and your phone could refuse to boot up. Any device with a locked bootloader on Android Nougat will use this new security. The good news is devices with unlocked bootloaders will remain the same.