Spaced repetition
Giving yourself a chance to almost forget something you’ve learned before testing yourself on it again will help you learn faster. The more times you’ve reviewed something, the longer the time periods between each review should be. Software such as Anki help automate the process by figuring out how long those time periods in between reviews should be. If you use Anki, you should know about Anki’s new scheduler: FSRS.
Flashcard apps are great for small pieces of information, but retrospective revision timetables may be better when entire topics need to be reviewed.
A closely related idea is interleaving: mixing up the order of what you learn helps you build more connections between those different topics.
Time after time, professors in mathematics and the sciences have told me that building well-ingrained chunks of expertise through practice and repetition was absolutely vital to their success. Understanding doesn’t build fluency; instead, fluency builds understanding. In fact, I believe that true understanding of a complex subject comes only from fluency.