The law states that when you sue a person, partnership, corporation, or the government, you must give formal notice to the other side that you have started the legal process. In the same way, when you are already involved in a case and file papers with the court, you are required to give the other side notice of the paperwork you have filed. The legal way to give formal notice is to have the other side “served” with a copy of the paperwork that you have filed with the court.
"Service of court papers" means that the other side must get copies of any paper you file with the court. In “service” a third person is the one who actually delivers the paperwork to the other side. The person who does this is called the “server” or “process server.”