The user interface is the layer that connects our application to the end user. One of the most challenging aspects of software development is making information meaningful and understandable to regular users of our application.

The UI layer serves exactly this purpose: it takes relevant information and shows it in such a way that it is easily understandable to humans. How do we achieve such a level of understandability with such a large amount of data? We use visual aids, such as colors, charts, and diagrams (just like how the diagrams in this book make the information easier to understand).

An important thing for any developer to understand is that your end user actually doesn't care about any of the backend stuff. The only thing that matters to them is a good experience. Of course, all the other components serve to make the users experience better, but it's really the user facing interface that leaves the first impression, and that's why it's so important to do it well.