“Service of process” is a legal term for the method by which a defendant is notified of an initial legal action. In the US, service of process is so critical that a lawsuit can’t begin unless service is completed properly. Generally, service of process is completed by delivering a set of documents to whoever needs to be served.
Every jurisdiction has its own definition of what counts as service of process, and the documents required can include an initial complaint or a formal summons issued by the court. In any case, the point is to notify a party that legal proceedings have begun and establish the court’s “personal jurisdiction” over all the parties involved in the lawsuit.
The important part is that Plaintiffs can’t serve process themselves. Process can be served, however, by anybody over the age of 18 who isn’t involved in the litigation. Usually, process servers are required to deliver the appropriate documents to an authorized recipient in person, by hand, rather than through the mail or in an e-mail.
Once process is served, the defendant is legally able to participate in the proceedings, including appearances before the court. If they fail to respond appropriately, the court can find them in default and ultimately award the plaintiff relief.