These are fractions that identify the rough milled thickness of lumber. You might be used to terms like 1×6 or 2×8 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:
“Woodworking 101: What Does 4/4 Mean In Lumber?”
