You might understand: How box layouts work. (I still don't)
You will understand: How flexbox layouts work.
The standard layout in HTML is weird. I still don't 100% understand it. I found even after many hours using standard layout I still sometimes struggled.
Flexbox is a newer way of laying things out that is far more sensible. I found after a couple of hours I could make any layout I wanted 99% of the time 😄
Couldn't find a perfect guide, these guides are pretty good but not perfect.
When something is prefixed with webkit- you shouldn't need to do that (that's how they prefix experimental webkit features)
Bare in mind which browsers flexbox supports