The essence of abstraction in software is that it hides implementation. The implementation is in some ways the opposite of the abstraction; where the abstraction is the gloss that describes how something can be used and what it will do, the implementation is the part under the covers that describes how it will work. If the gas pedal and the steering wheel are the abstraction, then the engine, power train, and steering assembly are the implementation.
Designers often focus on this abstraction of the system — the stuff users deal with. As a result, we spend a lot of cycles understanding users. But for the interface to be any good, designers must also understand the implementation — the system’s key elements, how they interact with each other, its processes, regulation mechanisms, etc.
Sometimes, as with a new (and perhaps unprecedented) system, this implementation itself is in flux, evolving subject to the system’s goals and the needs of the people who will interact with the system. That is, it’s not all front-end: the implementation is part of the design remit; both the implementation and its abstraction are the object of design.