This paragraph has these properties:
width: 450px;
height: 200px;
border: 10px solid blue;
padding: 1em;
box-sizing: border-box;

This paragraph has these properties:
width: 450px;
height: 200px;
border: 10px solid red;
padding: 1em;
box-sizing: content-box;

 

(Note the light-green border that is overlaid on the right paragraph. The light-green border shows the size of the same paragraph if it had the box-sizing value of border-box.)

(Also note that the left paragraph also has the box-sizing value of border-box.)