What is he talking about?
When you go to a microwave, is there anything you're really looking at besides maybe a few buttons? All I want is my timer, my time cook, the numpad, and the start button. Nothing else. When you are using a webpage, it is the EXACT same way. People are only concerned with the information that is pertinent to what they are trying to do. Everything else is just white noise for their skimming, and if you can't play to that strength, it is going to hurt the readability of your website.
Career Applications
Understanding that people skim webpages rather than analyzing every last detail is one of the most critical things I can take out of this course and into my career. We often are too proud of what we make, assuming everyone will think it to be just as important as we see it, but we live in a fast world, and we have to design with that in mind. Anything else will only lead to bad UX/UI design for our webpages and GUIs.
Credit
Krug, Steve. Don't Make Me Think, Revisited : a Common Sense Approach to Web Usability. [Berkeley, Calif.] :New Riders, 2014.