In CSS, the term "box model" is used when talking about design and layout.
The CSS box model is essentially a box that wraps around every HTML element. It consists of: margins, borders, padding, and the actual content. The image below illustrates the box model
Explanation of the different parts:
Content
The content of the box, where text and images appear
Padding
Clears an area around the content. The padding is transparent
Border
A border that goes around the padding and content
Margin
Clears an area outside the border. The margin is transparent