Opt-in
An opt-in is a person's explicit action agreeing to receive messages — commenting a keyword, tapping a button, or replying to a story. Instagram's platform rules are built around it: automated DMs may only go to people whose own interaction invited them.
Why it matters for creators
Opt-in is the line between automation and spam, and Instagram enforces it structurally: tools built on the official API can only message people who engaged first, and promotional links generally require a button tap (postback) before they may be sent.
For creators this is good news wearing a compliance costume. Messages that only reach people who raised a hand perform dramatically better — the opt-in filters your audience down to exactly the people who want the thing.