We use the word "serve" to talk about someone who uses their time to help other people. For example, someone can "serve" on the board of directors for a charity:
She's served on the boards of numerous charitable organizations.
A person can also "serve" in something like the military or police:
My family has served in our country's military for four generations.
Sometimes we say "serve in" and sometimes "serve on". It depends on what kind of group you're talking about:
- serve on the board
- serve on a committee
- serve on the police force
- serve in the army
- serve in the public school system