You work with a user interface (or UI) everyday, yes you. You can’t watch TV without using it’s UI, you can’t wash your clothes without using the washer’s UI, same with the microwave or your car dashboard, the Internet, or your mobile phone — all of these things have an interface that you interact with. Essentially this is what a user interface is;
The layer between you and the thing you are trying to use.
In a world where we are increasingly dependent on technology, what becomes important is how well that UI works — this is exactly what helps us to define usability.
For example, you may have the best car in the world, the most expensive desirable beautiful car ever made. However, if the designers put the ignition under the breaks for some stupid reason (to save money, because its trendy, etc) then you won’t be able to use the car; it will be useless. And 9/10 times this is exactly what happens because not enough people put us users first.
We are all users first.
If something is not usable it is useless, if it is useless then no-one will use it. This is the difference between choosing a car, a phone, a computer, a new app. It could not be more important yet it so often overlooked!