The Coase Theorem is a theory about how people and businesses can work together to reach a deal that is best for both sides. It is named after Ronald Coase who proposed the theory in 1960.
The Coase Theorem basically says that when two groups of people, or two businesses, or two countries need to make a deal to solve some problem, if they can discuss and look at all the costs and benefits, they should be able to make a deal that works out the best for both.
Basically, if two people are arguing about something, and both think they have the best solution, if they talk it through and look at what's best for both sides, then they should be able to come up with a deal that works for both of them.