I also talk about that a bit here

The mirror image rule is a traditional rule of contract law which requires an acceptance to contain the same terms as an offer, otherwise, there is no contract.

For example, let’s say Mr. A offers Ms. B his car for $5,000 and Ms. B says she accepts the offer but also wants the car to come with a big red ribbon on the hood.

The mirror image rule tells us Ms. B rejected Mr. A’s offer and made a counteroffer to buy a car for $5,000 with a big red ribbon.  Mr. A now has to decide whether he wants to accept Ms. B’s offer.  Mr. A does not have to sell the car and Ms. B does not have to buy the car until they agree completely on all the terms.

Here is an older video I made on the mirror image rule: