border-box tells the browser to account for any border and padding in the values you specify for an element's width and height. If you set an element's width to 100 pixels, that 100 pixels will include any border or padding you added, and the content box will shrink to absorb that extra width. This typically makes it much easier to size elements. box-sizing: border-box is the default styling that browsers use.
말 그대로 elem의 width나 height 설정 상황에서 이 값 안에 padding과 border값도 포함시켜줄 것이냐를 결정할때 사용하는 css 속성. (border-box는 포함한다.)
