These are fractions that identify the rough milled thickness of lumber. The actual size depends on if it's still rough sawn or if it's been surfaced.
| Rough Sawn (RGH) | Surfaced (S2S) | |
| 4/4 | ~1" | 13/16" |
| 5/4 | ~1-1/4" | 1-1/16" |
| 6/4 | ~1-1/2" | 1-5/16" |
| 8/4 | ~2" | 1-13/16" |
| 12/4 | ~3" | 2-13/16" |
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: