Reading Learn CSS - Flexbox
I believe it means everything is essentially on the same layer. so there are not things that are meant to overlap or appear behind. Its designed to create the best layout as long as everything is supposed to be laid out together within a layer.
The main axis goes in the line with the content so if it goes sideways the main axis is going side to side if the content is going up and down, the main axis is up and down. The corss line is always perpendicular to that.
It can flip content into an order that makes it difficult for someone to read your page.
Its easier to keep everything organized and to keep content from being pushed out of one side or another. Basically keep any one piece of content from out prrioritizing anpther.
I think I have some desire to work front end and I think CSS is going to be a big piece of landing a job in that area.