Being aggressive means to be pro-active in a situation. In tennis, being aggressive means to take the ball on the rise (just after the ball bounces), to go for winners, or to always look for opportunities to close points early. The opposite of being aggressive, or being passive, is to take the tennis ball late, to not go for winners, and to not close points as early as one can. In life, being aggressive or passive is relatively similar; being early or late, go for the win or not, and closing or not closing.
Being aggressive is a choice. Sometimes the situation is ideal to be aggressive and sometimes it's not. People tend to get better at reading the situation over time, after they've worked in multiple situations and have seen the pros/cons about being aggressive or passive.
Being aggressive all the time is much worse than being passive all the time. Being passive, or sitting back, allows one to take in the view much longer and scan the environment as long as possible. Being passive allows for more learning and more data points to be gained.
Being aggressive can be worked on like a muscle. You can train to be more aggressive or passive. If you notice you're always one way, try situations where you are the opposite of what you normally do. It may feel awkward in the beginning, but know that the more experiences you accumulate that awkwardness fades. The ultimate judge is whether the results are more beneficial or not.