Self-addressed means already labeled with your own name and address as the destination. A self-addressed envelope is one where you've written your own address in the spot where mail normally goes, so when someone else puts a stamp on it and drops it in a mailbox, it comes right back to you.

You might wonder why anyone would do this. Imagine you're entering a contest, applying to a summer camp, or requesting information by mail. The organization might ask you to include a self-addressed stamped envelope (often abbreviated as SASE). You write your address on the envelope, include a stamp, and send it with your request. This makes it easy for them to send you a response without paying for postage. They just stuff their reply into your envelope and mail it.

Before email became common, self-addressed envelopes were everywhere. People used them when ordering things from catalogs, entering magazine contests, or corresponding with pen pals. Today they're less common but still used for formal applications, legal documents, or situations where someone needs to mail you something back but shouldn't have to pay for postage.

The key idea: self-addressed means you've done the work of addressing it to yourself ahead of time.