To send for someone or something means to request that they come to where you are, or to order something to be brought to you. When a teacher sends for the principal, she asks someone to go get the principal and bring him to her classroom. When pioneers sent for supplies from back East, they ordered goods to be shipped to their frontier settlements.

The phrase suggests a certain authority or urgency. A doctor might send for a specialist when facing a difficult case. In historical novels, you might read that a king “sent for” his advisors, meaning he summoned them to the throne room. Parents might send for a child who's playing at a neighbor's house when it's time for dinner.

You can also send for things: sending for a catalog, sending for replacement parts, or sending for more information about a program. The phrase implies some distance or separation between you and what you need, and that someone else will help bring it to you. In an emergency, someone might send for help, meaning they send a message requesting assistance.