Android devices ran the check as part of the boot process and in Marshmallow, the phone would warn you that your phone was compromised but would let the phone boot.
Nougat says no.
According to the Android Developer’s bog not only does Nougat strictly enforces the boot check, giving you far more than a warning. Your phone will refuse to boot or will boot in a limited capacity mode - presumably akin to safe mode - until you can get rid of the malware.
Of course this means that some non-malicious corruption of data might also brick your phone.
According to the bog: “In the changes we made to dm-verity for Android 7.0, we used a technique called interleaving to allow us to recover not only from a loss of an entire 4 KiB source block, but several consecutive blocks, while significantly reducing the space overhead required to achieve usable error correction capabilities compared to the naive implementation.”
It might cause some problems for those who use custom firmware as any device with a locked bootloader will use this to check for modifications. This means that tinkering with your smartphone will be a lot harder with Android Nougat.