What is a reducer? A reducer is essentially a state changing function. It is called when we want to update or make changes to a state. they take two arguments. The state and an action. the state is passed from the store and the action is defined by you the programmer. An action is an object this 2 keys, the first is the type. This is the type of action you want to do which is used in your reducer to determine what your state should be. the 2nd is the payload or what you want the reducer to include in the state.