Being stoic is being calm and almost without any emotion. When you're stoic, you don't show what you're feeling and you also accept whatever is happening.
The noun stoic is a person who's not very emotional. The adjective stoic describes any person, action, or thing that seems emotionless and almost blank. Mr. Spock, from the oldest Star Trek show, was a great example of a stoic person: he tried to never show his feelings. Someone yelling, crying, laughing, or glaring is not stoic. Stoic people calmly go with the flow and don't appear to be shook up by much.