This stands for - Flexible Box module
The Flexible Box module, or just 'flexbox' for short, is a box model designed for user interfaces, and it allows users to align and distribute space among items in a container such that elements behave predictably when the page layout must accommodate different, unknown screen sizes. A flex container expands items to fill available space and shrinks them to prevent overflow.
To study flexbox container we must see how it looks
So the below image represents the elements in a flexbox in different directions -