Q:

What exactly does the flex tag mean?

I was wondering what the flex tag meant in css. Ive used it for some positioning a few days ago but I was wondering what it exactly does. 
Ive googled some explanations and camge to sites like W3 and W3schools but not more than that. And everything there was the same. 

Initial its: 1 0 auto

Most of the explanation is undersandable but I cant seem to find out what the "1 0 auto" means. As is now I still think its an boolean 1 = yes / 0 = no but can someone explain what the flex tag actually does? 
Ive used it in CSS like this
.flextest {
   width: 100%;
   height: 100%;
   display: flex;               <---
   align-items: center;
   position: relative;
   top: -81px;
}

This works for my problem but im not sure what I did here.
This was not a question here. So either this will be hated or this will be a good addition for the community. So I am sorry if this does not belong here. I saw it as an programming question.

A:

The initialisation of flex this way flex: 1 2 10%; means that you're setting specific values for the flex property.
`/* this is equivalent */
    flex: 1 2 10%; 

    /* to this */
    flex-grow: 1;
    flex-shrink: 2;
    flex-basis: 10%;
It is just a shorthand way.

A:

You can start by understanding the difference between flex containers and items.
display: flex creates a flex container. 

A flex container is the box generated by an element with a computed
  display of flex or inline-flex. In-flow children of a flex container
  are called flex items and are laid out using the flex layout model.
  Source

You can then customize how flex items behave within the container by setting various properties or using the flex shorthand property which allows the grow, shrink, and basis values to be set.

As is now I still think its an boolean 1 = yes / 0 = no

Correct, 1 = yes and 0 = no.
So flex: 1 0 auto (the initial value) specifies that the flex item should grow, not shrink, and be sized using the auto size basis.

