Flex

Resource Flex Layout 1

https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout

Resource Flex Layout 2

https://www.w3schools.com/css/css3_flexbox.asp

Resource Flex Layout 3

https://www.smashingmagazine.com/2011/09/css3-flexible-box-layout-explained/

Summary

There were even more resources I found that give you idea how to understand and use flex layouts. Some give visual example on what they are showing, which is more helpful to me and I am sure many more.