Clean Code refers to code that is not only understandable, but pleasing to the eye. As humans we all look for patterns in our world, and enjoy when things line up with our expectations. One of the fundemental piece of Clean Code is to make a pattern and stick to it. This is one of the main issues when working with a team that contributes to a dirty codebase. When each person brings their own personal styles to a codebase, not only can different files or components become confusingly different, but a single file could contain multiple different styles in itself leading to code with no recognizable pattern at all.
"The majority of people believe that programmers spend most of their time writing code, but in fact, they spend 80 to 90% of their time reading it rather than writing it." - Halim Shams
During both of my experiences as a peer tutor and as a mentor, it became invaluable to be able to read any code no matter how messy. But it also really enforced why I write easy to look-at code.
"Writing comes from reading, and reading is the finest teacher of how to write." - Annie Proulx
Top down abstraction is, in my opinion, a fundamental when it comes to writing clean code.