Android already has its security measures at startup of the device to ensure that there are no rootkits and other forms of secret malware. Since KitKat, Android has been working like this, which implies a bit of security for the user and thus avoid those problems that can sometimes occur with malicious software.
What it has not made a great effort to improve in this regard to Android 7.0 Nougat. In this new major version of Android, the OS force a boot check or start, even warning in a better way so that the user knows what to do. If it were the case that Nougat discovers that there is a corrupt boot image or partition, Android will start, but under limited use mode under user permission or not even start.
This major security feature will appear first on devices that are launched with Nougat, so the two HTC Nexus they are positioned as those who will see this special functionality first.
For the rest, this feature will become a great help by preventing your device from becoming the game table for all kinds of malicious software, at least since the terminal starts. The big problem with this measure is that third-party developers will find it more difficult to make adjustments with a custom firmware or ROMs. Google has mentioned that any device with a locked bootloader will use this extra to check for modifications.
So it will be quite difficult for modders to access certain touches that help improve those ROMs. Anyway, we will have to wait for Nougat to be released to see how much truth there is in it. Another one of those novelties from the previous 5 for developers like this from yesterday.