A more thorough explanation:

Service by mail is a way of delivering legal documents to the person being sued by sending them through the mail. This is done to make sure that the person being sued knows about the lawsuit and has a chance to respond.

In federal court, service by mail is allowed under Federal Rule of Civil Procedure Rule 5(b). This means that the legal documents can be mailed to the person's last known address, and service is considered complete once the documents are mailed.

For example, if someone is suing their landlord for not returning their security deposit, they can use service by mail to deliver the lawsuit papers to the landlord. They would mail the papers to the landlord's last known address, and once the papers are mailed, service is considered complete.

States may also have their own laws about service by mail. For instance, California Code of Civil Procedure § 1013 allows for service by mail, and states that the notice or other legal document must be mailed to the person's office address or place of residence. Service is considered complete once the document is deposited in the mail.

Overall, service by mail is a way of making sure that the person being sued knows about the lawsuit and has a chance to respond. It is an important part of the legal process.