body {
     font-size: 62.5%</pre> /* 1em = 10px */

The content inside the /* */ marks are CSS comments. This allows you to enter notes into CSS/PHP that will not be interpreted. In this case, this comment lets someone reading the CSS/PHP file know that that particular line of CSS/PHP was intended to allow for using ems to set font size later in the CSS/PHP in a more intuitive base 10 way.

Tip: when you add some functions in your functions.php file, add a comment ad the beginning and another at the end of your function in order to navigate in your file with ease.

Source: https://css-tricks.com/snippets/css/comments-in-css/