Ration bars are similar to energy bars, and are eaten by soldiers in the Horde. Ration bars come in two varieties - gray and brown, and they are the only sustenance that cadets and soldiers receive.
In Season 4 Episode 12, Fractures, Scorpia is confused when she's given a salad for the first time. This shows that some inhabitants of the Fright Zone may have never eaten anything other than ration bars, or ever even known about other types of food.
Both Adora and Catra like the gray ration bars much better than the brown ones. This is a bit of a running joke throughout the series, as neither option seems appetizing.
In Season 3 Episode 5, Remember, Kyle, Lonnie, and Rogelio make a "cake" out of Kyle's grey ration bars to celebrate Adora's successful first mission with the Horde. Adora says that the "gray kind is way better than the brown kind".
In Season 5 Episode 3, Corridors, during a flashback scene with Adora and Catra as children, Adora finds Catra hiding behind a stack of boxes in the hallway, and says "Catra it's dinner time, and you're missing it! It's the grey kind tonight, I know it's your favorite."