Computer networks. For years I was dumbfounded how IP addresses, VPN, ports, etc, all worked and tied together. Then, when I was interning as a software developer a fair while ago, a colleague drew the analogy of "IP address" = house on street, "port" = something you ask for when you knock on the door of a house.

Then it all just clicked. I still remember that day all these years later.

Other notable mentions, in no particular order:

* Perfect is the enemy of good enough. Did n't really appreciate this idea until around 2 years into my career.

* How to be professionally displeased at something. Early on my career I would get way too angry at incompetent colleagues peeing in the pool, e.g. bad code, design, management, etc. I would complain quite a bit! It only clicked ~2-3 years later into my career when I figured that one's displeasure at a situation should be a function of both how bad the situation is and how able you are to improve it. When you offer constructive solutions to incompetence (suggest alternative algo, management style, library, tool, etc) whilst not actually mentioning what is wrong, instead of just fruitlessly reminding people of what they did wrong, people become far more cooperative and receptive, etc.

My tin-foil-hat pet-theory is that the relatively recent tyranny of low expectations and "participation award" society has on average made younger people much more sensitive to negative comments about their work. The extension of that is that people end up robbed of more detailed reasoning about what they did that was wrong.

People are interesting!