The MDN website has a great explanation explaining what the CSS Flexible Box layout is and has an example to show you display: flex and the other properties.
CSS-Tricks has an easy to understand and complete guide on the CSS flexbox layout. The guide explains all the different possible properties for the parent elements and child elements. It also includes the history, demos, patterns, and a browser suppport chart to showcase flexbox. In my opinion, this website is the best guide visually to understand flexbox.
W3Schools has a quick explanation on layout modes before flexbox. Later on, they explain how to use the different flex container properties such as flex-direction, flex-wrap, flex-flow, justify-content, align-items, align-content. As you read more, they explain the flex item properties order, flex-grow, flex-shrink, flex-basis, flex, align-self. They also show you how to use them and what it would look like displayed.
In summary, CSS Flexbox has fairly great support and is a great way to layout a collection of items in one direction or another. Depending on how you are styling your website, you may choose to use CSS grid as well.