Even front end developers have more than 1 concept of an interface: there's both technical facing interfaces and customer facing interfaces.
I guess you could just give a generic answer: an interface represents some kinda boundary between users and implementation details, and hopefully said boundary is easier to use than the details.
I would guess some would flag that as a bullshit answer, but without clarification you can't do anything but speak in generalities.
Now if it were the interface keyword, they're primarily a means by which to introduce polymorphism. They no more achieve the goal of a generally-defined interface than does a regular class, which already satisfies the definition of the generally-defined interface through their public methods. This might also sound like a load of bullshit to some.