Purpose of * :: before, *:: after in CSS

Question

does anyone know what this CSS code is doing to the HTML? I'm not sure if I follow, so all elements will have 0 padding and margin, before and after they are inserted into the HTML? What is this doing to all HTML elements? And why not just have * {...}, instead of also having the ::before and ::after selectors?

  *,
  *::before,
  *::after {
    box-sizing: border-box;
    padding: 0;
    margin: 0;
  }