This is a small description of what is Flexbox.
Flexible boxes, or flexbox, is a new(1) layout mode in CSS3.
Use of flexbox ensures that elements behave predictably when the page layout must accommodate different screen sizes and different display devices.
For many applications, the flexible box model provides an improvement over the block model in that it does not use floats, nor do the flex container's margins collapse with the margins of its contents.
You could find more information on the W3Schools, and on the bible site.
Learn CSS3 Flexbox
Some games exist to teach you how simple is Flexbox :
(1) : New is not so true, first draft has been published in 2009. It works (partially) since Firefox 22 and Chrome 29 in 2013...