An interface is "common ground" for two different systems to work with each other when they know very little about what the other system is doing. When someone drives a car, the interface is the steering wheel, the pedals, and the gear shifter. You interact with this interface and then the system behind the scene works to help you drive the car.
Interfaces that are software related are known as API's - Application Programming Interface.
When a browser needs to interact with a server, we say that the server has an API. The browser has certain actions that it's allowed to take on the server and the server allows only those actions, that's the API.
Some API's are big and complex like a server, and some can be small like a function. For instance, when you do the rgb() function to produce a color for CSS, you provide certain input (arguments) in the required format and then you get some output from that. The output in this case is the desired color applied to the CSS property using that function. This function could be considered an API in that sense.