Furiosa: A Mad Max Saga explored the backstory of Furiosa (Anya Taylor-Joy) and some of the most important people in her life, among them Praetorian Jack (Tom Burke), and they stood out for wearing black grease on their foreheads. In 2015, thirty years after the release of Mad Max Beyond Thunderdome, George Miller brought the Mad Max franchise back to life with Mad Max: Fury Road. Set in a post-apocalyptic desert wasteland where petrol and water are scarce, Mad Max: Fury Road introduced viewers to another version of Max Rockatansky, played by Tom Hardy.

https://screenrant.com/furiosa-pratorian-jack-black-grease-paint-forehead-reason/