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.