what does box-sizing do in CSS code example

Example 1: border-box css

#example1 {
  box-sizing: border-box;
}

Example 2: box sizing

html {
	box-sizing: border-box;
}

*, 
*:before, *:after {
	box-sizing: inherit;
}

Example 3: box-sizing border-box vs content-box css

box-sizing:content-box;
	"Default. The width and height properties (and min/max properties) includes only the content. Border and padding are not included"
box-sizing:border-box;	
	"The width and height properties (and min/max properties) includes content, padding and border"

Example 4: how to style rule to apply the Border Box model css

header, ul, nav, li, a /* other elements */{

 display: block;
   -webkit-box-sizing: border-box;
   -moz-box-sizing: border-box;
   box-sizing: border-box;