“Flexbox is designed for one-dimensional content” means that flexbox is used to layout content either horizontally or vertically.
The main axis is the horizontal axis while the cross axis is the vertical axis. change direction with flex-direction
Using certain properties that rearrange the order of elements differently than how they are arranged on the HTML because screen readers will read them logically and not how they are visually portrayed.
You can do more with flexbox like justifying the content, changing the flex direction, making elements in a container have the same height even thought they have different amounts of content, and vertically aligning an element inside of its parent.
This topic opens the door to truly making a website look good, where you were kind of limited with floats. Using flexbox to move elements horizontally and aligning them I think will make my website look more professional and advanced.