What is flexbox in CSS?

The Flexible Box Layout Module, makes it easier to design flexible responsive layout structure without using float or positioning.

To start using the Flexbox model, you need to first define a flex container.

When working with flexbox you need to think in terms of two axes — the main axis and the cross axis. The main axis is defined by the flex-direction property, and the cross axis runs perpendicular to it. Everything we do with flexbox refers back to these axes, so it is worth understanding how they work from the outset.

See full guide at https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Basic_Concepts_of_Flexbox