There are so many great creators out there these day, I am sure that you would find in really easy to just type in a "____ maths explanation" into youtube and click on the first video that comes up. BUT BE WARNED: often dangerous oversimplifications and mistakes can be made with proofs by creators attempting to make ideas more approachable or clean.
I will recommend the following channels (which I am sure you will have heard of), who I can say safely are reliable.
I LOVE LOVE LOVE this channel more than I can say. With incredible visuals and a surprisingly soothing voice, this creator is able to introduce mathematical topics in incredibly intuitive and wonderful ways. As a mathematician, you MUST watch a least one of his videos (just go to his website and click a random one it's like a treasure trove!). As a non-mathematician, you should definitely still watch one - many don't require any mathematical knowledge and are lots of fun. This channel focuses on named maths topics, while others are more concerned with specific applications or individuals.
With stunning visuals, this website is incredible for geometric proofs of algebra for those looking for an intuitive insight into more abstract maths. These videos are much shorter than some of my other recommendations, so great for those looking for a quick dip in and out. Never have I ever felt so inspired by the beauty and simplicity of maths than I have been while watching these videos!
If you prefer watching people talk than just visuals and are more interested in the weird and wonderful of applications of maths, try Numberphile! With both podcasts and videos, you will be sure to be enthralled by mathematical history AND kept up to date on all the latest breakthroughs through their guest speakers from around the world. Video journalist Brady Haran is a wonderful example of a non-mathematician interested by maths.
This site is more on the computer-ish end of maths, focusing on algorithms and hence might be a better recommendation for those with a bit of mathematical experience (I am not talking more than GCSE). It is based on the Arcane Algorithm Archive, which I will do a post on separately. This is a collaborative effort to create a universal guide to all important algorithms. The videos are an amazing way to enter into computer science, and move quickly into some incredible content!!
The creator is really friendly, and you really feel like you are a part of a great community when you watch his videos. Much like Think Twice these come in the form of shorter, bitesized videos.
There are many more, often which come with written resources for more complete learning. However, I'll save those ones for another time!
Which youtube channel is you favourite? Got any more recommendations?