Flex question

I am on MDN at this link trying to figure out flex…

https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Basic_Concepts_of_Flexbox

1.) flex-basis

What does “0” represent?

What would a poistive number represent?

2.) flex-shrink

I do not understand how this works. I am playing around with the live code sample under “Shorthand value for the flex properties” and I just don’t get what the middle value does.