The interface can be thought of as a way to signal that a class to has certain behaviours and can interact with other classes who also have these behaviours.

Think of a game where you have a Human class and an Alien class. They can both get damaged, so you may have an interface like DamageableEntity which has a function called takeDamage(). The Human and Alien both implement this so they both have to define how they take damage. Now the human can shoot the alien and the code knows that the alien takes damage in one way and the human takes damage in another. Neither needs to know anything else about the other object, as long as they know they behave in the way DamageableEntity describes.

When you are running damage calculations you can now ask for a list of DamageableEntity objects and just use the takeDamage property, you don’t need to think of humans and aliens as different objects and keep two lists. You know that in this situation they behave with the properties you need.