An OPT-in clause is a type of agreement between two parties that allows one party to give permission for the other party to do something. It is often used in marketing and advertising, where a company may ask customers to OPT-in to receive promotional emails or other communications. It is also used in legal contracts, where one party may agree to certain terms and conditions in exchange for something else. The term 'OPT-in' comes from the fact that the party must actively choose to agree to the clause.