What is service of process?
Generally, it is the procedure of finding and informing a defendant of your claim against them. Thankfully in our nation, you must be duly notified of a court proceeding against you. In the United States Constitution, courts cannot exercise personal jurisdiction over a defendant unless notified. Until that time, no action will be taken outside of locating you. This is a process that is simply defined and complicated to execute.
This is a complex civil procedure carried out in a few different ways. A defendant must be physically notified (an agent directly serves you), by certified mail, by a court agent or notifying court instrument, or in the rarest of scenarios by email.
There are ways to waive service, but it is still considered a notification. This only provides the defendant with more time to prepare a response. However, if you refuse, as the defendant you must reimburse the costs for arranging in-hand service. This can be a few hundred dollars depending on how difficult it was to locate you.
If you are the plaintiff, certain defendants will try to evade or actually be difficult to locate. In order to find the defendant, you perform a procedure called Skip Tracing. To simply define it, by using information like a social security number the detective searches public information and physically discovers their location. It has been a rather successful system.