Here's a breakdown:
* "mas" means "more"
* "madilim" means "dark"
So, "mas madilim" literally translates to "more dark," which means "darker."
Here are some examples:
* Ang kuwarto ay mas madilim kaysa sa sala. (The room is darker than the living room.)
* Ang langit ay mas madilim kaysa dati. (The sky is darker than before.)
* Ang kanyang buhok ay mas madilim kaysa sa akin. (His hair is darker than mine.)