The 80/20 rule is simple. 80% of results come from 20% of actions (and hence also the other 80% of actions bring in only 20% of results). As it applies to most things, it probably applies to many parts of your life too.
The 80/20 rule means that we need to review our lives every now and then, and cut out actions that are done out of habit, but really don't contribute much to our success or wellbeing. This may include habitual actions and activities, toxic friendships where the other party really isn't interested anymore, or even an unattainable long term goal where it would make better sense to quit rather than to invest more. 80/20 rule reviews are painful, but are worth it in the end.
However, we shouldn't get too obsessed with the 80/20 rule, for example by applying it continuously in life, even pre-emptively judging any new activity or relationship by it. New activities and relationships, new goals in life and new commitments all have three things in common: 1) the likelihood of success cannot be determined right away, or even perhaps in the first year; 2) the first investments are often the most difficult and come with the least return; and 3) if you don't get fully into it and try your best at it, you aren't going to know what you can get out of it. Therefore, those who apply the 80/20 rule too soon often quit before they get any returns. It is best to not apply the 80/20 rule until patterns have become settled, which usually takes a year or more.
In conclusion, the 80/20 rule is very useful to go by, and can clean out much dead wood in our habitual lives. However, to apply it to new commitments would be a grave mistake. Like everything else, there is indeed two sides to this coin.