The basic premise of 80/20 is this: the overwhelming majority of what goes into your body (80%) comes from food that will nourish your body and the remaining 20% is reserved for foods that may not be nutritious by any means but nourish your soul-- think the bowl of gelato you treat yourself with for a job well done.
There's no need for 80/20 to be some complicated undertaking. The goal is to go about this intuitively, listening to what your body needs and finding a healthy balance that works for you. Personally, I aim to keep things pretty clean during the week-- 3 nutrient dense meals with a treat after dinner-- and splurge on the weekends with a nice dinner out followed by some kind of dessert.
My 80% consists of foods that I know my body thrives on: an abundance of vegetables, herbs, avocados, free range eggs and fatty fish (salmon is my go-to). Whereas the remaining 20% of my diet is composed of sugary treats and high carb goodness like bread, pasta, cookies, cookie dough, gelato, etc..
It's important to note that there are no "bad" foods, just foods that our bodies don't run optimally on. This is largely variable from person to person. For instance, I don't do well with sugar and need to be careful even with natural sugar that comes from fruit. Because of this, fruit occupies my 20%, not because it's an unhealthy food group but because it doesn't work well for me when I overconsume it.
I view 80/20 as a way to find a balance between nourishing my body and living my life. It's a relaxed way to remind myself to eat my broccoli but also to enjoy the slice of cake waiting for me at the end of my meal.