MDN explains CSS Flexible Box Layout well and has good examples to showcase it.
I personally prefer w3schools because they simplify the examples making it easy to understand.
This page definitely showed a lot of examples and also showed how all the properties work and how it would look like. They really go into depth about the CSS Flexible Box layout module.
Flexbox is designed as a one dimensional layout model for arranging items into rows or columns. This method can offer space distribution between items in an interface as well as fill space or shrink items to fit into smaller spaces. It involves many properties and is usually a better alternative than floats.