The local database used by Signal to organize every message, every contact, every profile photo, every attachment, every group, basically every dynamic piece of data you interact with in the app.

Signal is basically a UI layer for a database. The in-transit encryption is genuinely good enough to be textbook study material for cryptographers, but the at-rest encryption became a joke the moment they stopped using your pin to encrypt the local DB and requiring it to open the app.

As someone who's been enthusiastic about Signal since it was TextSecure and RedPhone, the changes made over the years to broaden the userbase have been really exciting from an adoption perspective, and really depressing from a security perspective.

TL;DR of Molly is that it fixes/improves several of those security regressions (and adds new security features, like wiping RAM on db lock) while maintaining transparent compatibility with the official client.