On w3schools they give you a summary of what css flexbox is and how it works. They provide the browsers that support flexbox, they talk about flexbox elements, and they layout. They also provide examples after every area.
On smashingmagazine there is more information and code that you can look over. There is a list of all the properties and how to use some of them. After the code example there is a image of what the code would turn into.
On developer.mozilla there is a summary of the flexbox layout, then they provide a basic example to show you want it would look like. They provide all of the properties and a link that will bring you to a page with more information about the property.
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.