Flat affect means that if your emotions are happy or sad or calm or hungry, your face remains the same, more or less.

Flat Affect – A Visual Guide

Flat Affect is when your face, tone, and other involuntary non-verbal communication cues don’t quite do what they’re expected to do (for example give insight into an emotional state). If you have flat affect, you may be accused of being emotionless, cold, or overly sarcastic. You might actually be these things, but also it’d be nice to have the option.