Google ran into a problem -- slower processors used in less expensive smartphones for emerging markets couldn't reasonably perform full disk encryption in order to protect user data. So, Google created a solution and gave it a name that doesn't really help you understand what it is (which is a common fault of Google's product names): Adiantum.
The basic problem is that although ARMv8 processors have hardware acceleration available to make the traditional AES encryption standard work quickly on higher-end devices, ARMv7-based chips don't have that functionality and so processor performance would take a huge hit when dealing with encrypted data. Without getting too deep in the technical weeds, Google claims to have solved the problem by using a combination of two encryption ciphers -- one of which is the basis of HTTPS traffic -- in order to be able to securely encrypt data without causing a hit to performance.
Google's new encryption mode, called Adiantum, is approximately 5 times faster than AES on chips that don't have hardware acceleration support for encryption, but should also be secure enough to be extremely difficult to crack (at least until quantum computing is ready for the spotlight.)
Unfortunately, Android Pie is required for Adiantum and manufacturers will have to turn on support for the full disk encryption, so we'll have to wait and see what the rollout looks like.
source: Google Security Blog