It is a box that wraps around every HTML element. Its mainly used for design and the layout of a website.
The padding goes around the content, like this element paragraph. While the margin is the space between this content and the next element.
The height and width in html are determinded by the person making the website or they are set to fit the whole page. To set the width and height you use px (pixles) to determing the size you want or you can use the % sign which sets it to the size of what the screen is set too.
Padding is the surrounding of the text. Right now the padding is set to 25px by 50px. This is the dimentions that the text will fit within.
Borders sit around the text and makes them pop within the text. While outlines make all the text pop and easier to read depending on the background.