Lack of understanding would mean you haven’t decided meaning from the words on the page. He gives the example of memorizing a German textbook without speaking German.

Memorization without learning means that you don’t know the relationships between individual facts that give them meaning and relative importance that lets you make wise decisions about what to memorize and what not to for your purposes.

So: make sure you understand what the words and phrases you’re reading mean (understanding). Look up terms and definitions. Identify the main points of the sentence, paragraph, page and chapter and why they matter (learning). Then memorize those main points, starting with the most important basics.

Memorization is part of learning. Memorized knowledge is knowledge. But memorized knowledge isnt understanding.

You can fully understand something without being able to recall it perfectly later.

Not sure if this will help (or if it's even correct), but Wilfrid Sellars in his essay "Philosophy and the Scientific Image of Man" talks about the difference between "knowing that" to ride a bike one needs to put pressure on the pedals to rotate the tire and move the bicycle and rider forward and "knowing how" to ride a bike. To me the latter is indicative of understanding and the former is information or knowledge.

Understanding is prior to learning. Learning is required when prior understanding fails to extend/generalize/transfer. Understanding is a residue of learning.

Mortimer Adler's How To Read A Book talks about "reading for understanding" a lot. By understanding, he means getting a holistic feel for the content. Grokking, if you will. An analogy is reading a newspaper article on what happened in Gaza yesterday versus reading a 500-page book on the history of Palestine and writing a report on it. In the first example you are reading for facts (knowledge), in the second you are reading for understanding

How does one walk? To walk one needs to move their foot forward, to move their foot they have to lift it up, to lift a foot up they first have to place it down, and that requires moving it forward. It feels circular!

Learning is circular. You do it step by step, one bite at a time: you learn a fact, you understand its connection to other facts you know, you gain a little knowledge, you repeat.

You are correct that those concepts are interrelated. It works well not to get caught up in precise definitions. Instead, reflect on your current level and take the next best step.

Enter: knowledge. It's a messy thing. Before you know it, it 'clicks'. That's the only thing worth chasing.

There is a lot of fetishization of this terminology in the spaced repetition community. It's really best to ignore it, it's not based in much.

I think their mental metaphor is that cards allow you to memorize nodes, and understanding is having a feel for the entire graph. But cards also help you to memorize links between nodes, subgraphs, overviews, principles, etc...

I also think it's mostly a ready made array of excuses to read off to somebody who is having a crisis of faith about whether a Anki is helping them or not: you're holding it wrong. You haven't put in the work. Are you making your own decks, you can't use other people decks because making your own decks = understanding (for mysterious reasons, do you really understand something you can't remember?) Are your facts atomic enough? Basically direct or indirect paraphrases of the Supermemo wiki.

Supermemo didn't discover anything, he computerized something that desperately needed to be computerized, and at that point wasn't restricted to the algorithms that could be executed by shuffling around physical cards, such as Leitner boxes (which are awesome, still, by the way.) His analysis is great to read and often insightful, but is no more profound than many others and often far less scientifically grounded. People just are addicted to parasocial relationships with self-improvement gurus.