Service is giving notice to defendants about a court proceeding or trial date. When you file forms with the court, you are considered the plaintiff. The other person or people named in the court forms are the defendant.
As the plaintiff, if you don't let the defendant know about the court proceeding, the judge can dismiss your case.
For example, if you file for divorce, you have to let your spouse know that you've filed for divorce. Another example includes changing your child's name. Even if you don't know where the other parent is, you still have to let them know that you're changing your child's name.
There is a time limit for service. You have to give service to the defendant within 120 days (approximately 4 months) after filing your court forms. If you don't, then the judge can dismiss your case.