If you opt in, you choose to accept something, or do something, that is offered to you as an option.
🤓For example ⤵️
💬opt in When they're purchasing a product online, we give our customers the chance to subscribe to our newsletter, but we only send it to those who've opted in.
💬opt in We have English classes for our workers. They don't have to attend, but if they opt in they're expected to complete the course.