Flexbox is a CSS module that aims to provide a more efficient way to lay out items in a container, even when their size is unknown or dynamic. It allows items to align and distribute space flexibly. Browser support for flexbox is now widespread, though it was previously complex and different than traditional layout methods. Flexbox offers properties to define flex containers and items for controlling direction, wrapping, order, growth, shrinkage and more. It can be used to create complex nested layouts.