These are fractions that identify the rough milled thickness of lumber. You might be used to terms like 1x6 or 2x8 for softwoods (pine, fir, etc), hardwoods are measured differently, starting with the thickness in fraction form.
The fractions read in quarters of inches. For example, 4/4 is called "four quarter," and it means approximately 1" in rough thickness.
You might find this article in our blog pretty useful if you want to know more: