Easy things feel fast, hard things feel slow, but in the long run you'll get more from doing the hard thing.
In the context of language learning, they were comparing apps like Duolingo to WaniKani (for learning kanji). In summary, your brain can sucker you into thinking it's learning something, when in reality you're not being challenged at all. Doing 100 multiple choice in a day can FEEL like you're learning a lot in a short amount of time, but picking an answer when it's in front of you doesn't test your recall. Being forced to recall 10 kanji from memory over 10 days will help you learn them more reliably.
I think this lesson extends to almost any domain. You can fall into a trap thinking "I understand how to do X in theory, but actually doing it will take so much longer. It would be a waste of time for me to do something I already understand how to do." But in almost every case where you've thought that, once you dig in and actually do the thing, you will hit roadblocks that you have to work around, you'll feel discouraged and confused about why it wasn't straight forward, you'll commit things to memory because you're forced to, and when it's all said and done you will have a much deeper understanding than if you had optimized that task out.