The base text is set to 100%, or 16px, with a line-height of 1.5, or 24px. These become the base units for font-size and line-height. The main heading above is set to 300%, or 48px, which generates a line height of 72px, 48 * 1.5 = 72. The 1.5 line height is inherited from the base text, which is set using the body tag as a selector.
The above subhead (h2), is set at 150%, or 24px. To keep it aligned, the line-height is set to 1 (flush), or 24px, which is one line height from the base. An additional line space is given by using a margin-top of 1em, or 24px, which is in relation to the current h2 font-size of 24px.
The first subhead is fine, and works, and stays aligned, but it is too close to the following paragraph. So, rather than using the entire line space (as a margin) at the top, the 1em can be split between the top and bottom margins.