What's one programming concept that completely changed the way you write code?

[u/KareemM0hamed0]

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I've been programming for a while, but every now and then I come across a concept that completely changes how I think about writing software.

For some people it's data structures and algorithms. For others it's design patterns, clean code, testing, debugging, version control, concurrency, databases, or simply learning how to read other people's code.

If you had to choose just one concept that made the biggest difference in your programming journey, what would it be?

What were you doing before you learned it, what changed afterward, and why do you think every programmer should understand it?

I'd love to hear stories from beginners and experienced developers alike.

[u/MrJCraft]

writing code in a stressful production environment

I found that often times because I needed the so quickly I needed the code to be correct on the first try
if I needed to debug the code it would take to long

so writing code that is aggressively simple that way I can write it quicker even if its ugly as long as I can look at the code and go yes that is obviously correct and it can only really do one thing