It's important to understand "every scrap of code" in this context. Think about reading a novel: is it essential that you know every single word in the book? No, of course not, most readers will come across a few words that they're not familiar with in a novel, or perhaps some words that they know but which are used in an unfamiliar way. That's fine; you can often get the meaning from context, and you can look it up if you need to. But degree is important: if there are too many words that you don't know, it can be difficult to follow the narrative and looking up definitions too often gets in the way.

It's much the same with programming. You need to understand enough of the language that you can follow the code without too much trouble. Programming is all about defining new things: functions, classes, methods, constants, variables, etc., so a lot of understanding comes from digging into code and documentation, sometimes several layers deep, to figure out how things work. Every language has features that aren't used often, and if you're not familiar with all of them when you start you just look them up as you encounter them. But if you're not well versed in the main features of the language, the parts that are used frequently, it's going to be difficult to understand the code and even more difficult to write new code.