Sorry to ask this--it's surely a FAQ, but the nature of the question makes it impossible to find existing answers with normal search functions!
More than once I've seen something like the following in a copyright page or colophon. This example is from a copyright page:
"Text set in Janson 11 on 14."
What does the "11 on 14" mean? I believe I recall seeing "n point on m point" or something similar, so I now believe that the numbers are point sizes.
(Wild guess: The first number is the font size, and the second specifies the amount of leading. Maybe "11 on 14" means that there are 3 points of extra space?)