A mutation in GraphQL is a way of modifying data on your server. You can define a mutation in your Ruby on Rails application by adding a method to your schema file. This method should have the same name as the mutation you want to define, and it should take arguments that match the data you want to modify. It should also return a type that matches the shape of the data you want to return after the mutation is performed. You can then use this mutation in your client application to modify data on your server.