I had the same problem as it seems the convention isn't explained on the website. However as soon as I took a look at the code it became clear that the numbers

I had the same problem as it seems the convention isn't explained on the website. However as soon as I took a look at the code it became clear that the numbers represent fractions. .col-2-12 is a column that is 2/12 in width (same as 1/6) and .col-10-12 is 10/12, respectively.