Maybe I'm so far behind on my maths education (stopped at 18 after UK A Level) but I found a lot of 3Blue1Brown's videos hard to understand. It's possible that he has prerequisites I never watched. YouTube is bad at discovery.
So don't be discouraged if you need to pause often or take a long time to finish the video. The rewards are worth it.
For example, the 3b1b linear algebra series is great but I couldn't have used it to become proficient at _doing_ linear algebra. Even having taken a linear algebra class, I got more out of that series as a supplement to actually working out math problems on paper than as a primary source—and I definitely think that's what's intended.