Specific gravity is the ratio of the density of wood to the density of water.
A wood doesn't float, for example, if it's specific gravity is 1.00 or greater.
We display the specific gravity for each of our woods using it's kiln-dried measurement (instead of the non-dried measurement).
It's Just a Guide
The trouble with this number is that the specific gravity of a wood changes with its moisture content. So use this number just as a ballpark guide to estimate how dense a wood is. We've also compared the number to that of red oak, as red oak is the most commonly used hardwood in North America.
Why Does It Matter To You?
Most of the time, the higher the specific gravity, the more abuse your tools take. Conversely, woods with low specific gravity don't make good choices for furniture and are more difficult to create nice, strong joints with.
Just use this as a guide to help you make an informed choice.