https://en.wikipedia.org/wiki/Empirical_formula

In chemistry, the empirical formula of a chemical compound is the simplest whole number ratio of atoms present in a compound

In this comic, the "compound" is the entire observable universe. So it's the simplest whole number ratio of every atom in the observable universe

A more in depth explanation for this comic (and all other xkcd comics) is here: https://www.explainxkcd.com/wiki/index.php/3200:_Chemical_Formula