Free Code Camp
freecodecamp.org offers a flexbox tutorial with definitions, a cheat sheet for flex properties and values, and code projects.
Josh Comeau
joshwcomeau.com is much nicer than freecodecamp. He has definitions and working examples on his site so you can see the affects in realtime. You can change the properties and watch the change before your eyes. His site also has the dark design that I love. I bookmarked the page, and I'll definitely be going back.
codeinwp
codeinwp.com is very similar to the joshwcomeau example above. It has pretty detailed examples and definitions as well as interactive samples.
Summary
In summary there are a lot of flexbox tutorials out there. For the most part they are very similar in regards to content. The only thing that would drive me to joshwcomeau over codeinwp.com is the design.