MDN web docs

MDN web docs explains how to use flex in CSS Flexible Box Layout, states its properties(flex, flex-basis, flex-direction, flex-grow, flex-flow, flex-shrink, flex-wrap, order), properties for alignment(justify-content, align-content, align-items, align-self, place-content, place-items, row-gap, column-gap, gap) and provides various guides about the topic(concepts of flexbox, relationships of flexbox to other layout methods, aligning items in a flex container, ordering flex items,ratios and wrapping of flex items etc.).

W3C

W3C contains the official set of rules about using flex property with details. There are introduction, terminology, flex xontainer, flex items, ordering&orientation, flexibility, alignment and flex layout algorithm.

W3Schools

W3Schools has a short description of CSS Flexbox Layout Module, shows browsers support and gives a simple flex example.

Summary of the documentation

In conclusion, Flexible Box Layout allows to put children of the flex container so that they either fill unused space or shrink not to overflow the parent both horizontally or vertically. There are four Flexbox Layout modes: block, inline, table and positioned. The Flexbox Layout Module makes it easier to create responsice layout sructure without using float or positioning.