How much code in your game do you actually understand?

There are different bits of code and shaders that I find online that work and I try to understand as much of it as possible, but sometimes when something works and I need to move on to other parts - there's just no time to figure out why the code does what it does, or I just don't have the brain power. Is this normal or am I a terrible developer?

Edit: Thanks everyone for a great discussion. I wish I could reply to everyone. Had no idea Godot subreddit is so responsive.