I'm unable to understand padding concept.
Picture a box. When we clear the space around the box, that is called margin. By clear I mean set. We can establish a comfort zone for our box.
Now put yourself inside the box and think as though you are not allowed to go anywhere closer than a set distance from the walls of the box. Your size may grow, as in "Thinner", and so will the box, oddly enough. But you will never be able to reach closer than a given distance from the walls of the box. That is padding.
Think of padding as your content having the same electromagnetic pole as the box itself, and so they repel one another.
lets say you are reading a book, now lets say one of your pages is a box, as you will see, there is some spacing between the edge of your page, and the the text. You could call this padding, spacing inside the box/page