1. The CSS box model is referencing the layout surrounding each HTML element. There are 4 parts to the box model which are the margin, border, padding, and content. These sections allow more customizability to the spacing around things, as well as letting you modify a bunch of different aspects of the "box" to make it appear how you please.
2. The difference between the margin and padding is that the margin is the exterior around the box, outside of the border, while the padding is space between the content (text or image) and the border on the inside. The margin being changed would affect where the entire box is located, while padding will determine where the content is located inside of the box.
3. The height and width of the HTML element depends on whether you are leaving it to be determined automatically based on the type of content/wrapping settings, or manually set the width and height yourself. You can customize how it determines the position with the "position" property, which can move the content to be in a static position (default), an absoulte position, fixed, relative, etc.
4. Setting the padding with "padding: 25px 50px;" results in a padding of 25 pixels above and below the element, and a 50 pixel padding to the left and right of the element. This is all spacing between the element and the border of the box, and affects the wrapping within the box, not the margins outside the box.