The reader should predict the definition so as to know what the word means, at least vaguely. If not, he/she should consult a dictionary.
Romeo says this while talking to Benvolio (who's crying). Romeo is sad and Benvolio is being sad too because Romeo is. Romeo is explaining to him what love is to him and why its making him sad.
The quote "more like a king, composed, farther and farther on" has a very important meaning. It is saying that you should, rather than be shy, or to not have pride in one's self, instead compose yourself "more like a king". It is saying that you should be proud, and that you should carry yourself in a way that makes others admire you, because you have pride in yourself.
Can you please provide the passage?