Three Blue One Brown (3b1b for short) is an excellent math explanation channel by Grant Sanderson with beautiful visuals, great for building intuition. I found that personally the calculus and linear algebra courses have helped me in high school and college.
Great YouTube channel out of the UK where they give mathematicians sharpies and butcher paper and they explain their research in simple terms. This channel is much more on the entertainment side then any other channel I've posted here.
Richard Borcherds is a field medalist who uploads videos on undergraduate and graduate mathematics with clear explanations. Sometimes, he is a tad fast when he speaks. However, there is no substitute for what he offers online. I have additionally found this website to be very useful in my studies at college.
Matt Parker does wonderful videos of himself, somewhat comically, explaining math concepts that come up. He is distinct from 3b1b since he is filming himself and occasionally uses props, unlike the generally purely digital ways in which 3b1b makes his videos.
Here are some of the video I enjoy from the respective channels: