Context: This is when somebody doesn’t react/listen/change no matter how much they are insulted. The other party asks if they add salt to their food, or if they only eat bland food, and thus have lost all emotions and have become as bland as their food.
It’s a bit difficult to explain, but the general belief is that food reflects your emotions and reactance and moods. Bland food - emotionless, spicy food - easy to anger, etc.