A transaction could fail when Transaction gas volume exceeds the block gas limit. But in this case it was a complex transaction which required huge gas and that 21000 gas was not sufficient for the transaction to complete. He should have used 20gwei as price per gas and should have used very high gas volume which could have made the transaction pass.
You may not end up in such a situation as high gas volume are very rare and if you are doing a normal transaction just use 21000 gas and 20gwei gas price for it to conform. The base theory of ethereum transaction is similar to a taxi ride you are going mate. If you need to travel longer distances use higher fuel how complex the route might be. Our routes are simple mate which require 21000 gas so don't worry.
Hope this answer helps
Thanks so much. This really cleared things up! So basically use Ethgasstation, and also there's a hugedifference between gas price and gas limit... and the guy decided to use the same for both and so it's like he was trying to use a bigger car engine but he didn't use enough gas because he also increasedid gas consumption,