The CSS box model is a fundamental concept that defines the layout structure of an element. Each element is represented as a rectangular box, with four main components:

• Content: The innermost part where text and images are placed.
• Padding: The space between the content and the border. This adds space inside the element.
• Border: The line around the padding.
• Margin: The outermost space that separates the element from surrounding elements.

By default, the box-sizing property is set to content-box, which means that the width and height of an element apply only to the content, with padding and border adding to the overall size.

When you use box-sizing: border-box;, the width and height include the padding and border, which prevents the element from becoming larger than expected when you add padding or border.