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