If you're familiar with all the properties that comprise a flexbox layout, you're probably thinking "holy crap that's a lot of new CSS properties I have to remember". Indeed it is, but Flexplorer can help you learn them and even generate some of the code for you.
If you're not familiar with flexbox yet, here's a primer by CSS-Tricks. I recommend at least skimming through that before trying out Flexplorer so the concept will be a little less perplexing.
In short, you can play with various flexbox properties and the tool shows you a live preview of the resulting layout. You can change the number of items, direction, wrap, etc. This is a great way to visualize and create layouts for your next website, should you decide to start using flexbox. (Just keep in mind that it isn't supported in IE10 and below.)