In Object-Oriented languages an interface defines a set of methods which a Class must include in order to implement the interface (otherwise, if the Class is missing the required methods, the code will fail and the interface will throw an error).
Interfaces are useful for making sure developers use the correct implementation of an API.
In JavaScript there are no true ‘classic’ Object-Oriented features, but through clever usage of the language you can emulate an Interface for use with a JavaScript API.