Definition of legible
1: capable of being read or deciphered
2: capable of being discovered or understood
murder sweltered in his heart and was legible upon his face
The first one you know. UI/UX/Design stuff. Being easy to read. But the impact, positive and negative, of making things legible, especially the second definition, runs way deeper than choice of font size and foreground/background color.
Code can be readable and completely illegible. Green text on a black background with a monospace font that makes it easy to distinguish between 1 (the number one), I (the capital letter `eye`), and l (the lowercase letter `ell`) will make your code readable. But it doesn’t do much to help with discovery or understandability.
At the simplest, legibility in code comes from clean code. Separation of concerns. SOLID. KISS. DRY. All those acronyms. If you do those things reasonably well your code will be reasonably legible. At least at the tactical level.
But having truly legible code goes way beyond that. It’s about applying the same principles you would apply to a module/library to an entire system. It’s about your abstractions and data models and APIs. It’s about making sure that the system is understandable/discoverable at both the large and small scales, and that it’s easy transition between the levels as needed.
One thing that’s important to keep in mind while making things legible is that your model(s) of the system need to truly match reality, not just how you want reality to be. Take a complex system, make some simplifying assumptions, idealize things, and make it happen. When you do that it often feels correct, because you have control over what you’re doing. It’s predictable, understandable, and subtly wrong. But you won’t know it at first. It will mostly work. Until you hit that edge case.
So you patch around it. Until the next edge case. Rinse and repeat. Pretty soon your simple, elegant, legible system is none of those. So you come up with a new model and try again. And that cycle repeats.
Unless your models acknowledges that things aren’t that simple. That they allow for unexpected interactions. And that’s hard. Especially in large systems.