The 20/60/20 rule is as such:

20% of the games you play you will lose by factors completely outside of your control. Whether this be smurfs on the other team or teammates who are just having a bad game or teammates who DC, some things are just out of your control.

60% of the games you play can be swayed based on how you play. These are the games that are closer (11-13), or you're the person on your team having a bad game and weighing your team down. These are the games that you should be worrying about, the games that you can look back on and improve on. These are the games that will make you a better player in the long run.

And finally, 20% of the games you play you will win by factors outside of your control. Similar to the previous 20%, sometimes you have a teammate who is popping off, or a smurf on your team, or your enemies are afk. These games you essentially get a free win.

Why does this matter? People get way too caught up in blaming teammates for losses or complaining about the other team smurfing when the odds of the reverse happening (the enemy doing poorly or your team having a smurf) is, roughly, the same.

If you want to rank up and get better at the game, you have to focus on the losses you have in the 60% of the games you can control. Start recording your games, go over the games that ended up really close and see where you went wrong, see what you could have done better.

And also, this isn't meant for people to automatically chalk every loss to the 20%, of course some stretches of time will be more loss heavy or more win heavy (whether or not this is from the 20%'s is debatable), but focusing on what went wrong in a game is the start to becoming a better player

EDIT: A lot of people in the comments seem to not realize the actual point of the post. It's not about the exact percentages, it's about the mindset that you should have when going into games and how you should look back on games after the fact. Of course these numbers aren't accurate and some people will have different experiences (especially in small strings of games), so take everything here with a grain of salt :)