It’s a custom Android ROM focused on security, it does what many Android custom Roms are failing to do, which is bootloader relocking on a non pixel device, it also has other neat enhancement but I’m not gonna list them all here
They also maintain other awesome apps like : Mull ( a hardened fork of FF mobile ), Mulch ( a hardened fork of Chromium mobile ), Hypatia ( Android Antivirus )…etc
See this post to understand what’s happening
Maybe Kanji dojo if you wanna learn Japanese, And trivia you to quiz yourself