The tool is only two identical plastic plates that are held together with a small carriage bolt and a knurled brass knob. Each side has a scale and a rack of grooves that engage the matching rack on the other plate. The racks lock together and precisely lock the tool at your desired dimensional setting. This is great because you aren't left trying to eyeball your dimensions while you measure and set your tools, but rather you have a physical item that is locked at your dimension. I know I'm doing a poor job explaining this, but it's just a neat and simple tool.
Here's a link to a pdf of the owners manual for the Incra Gauge, and here's a link to the tool at Woodcraft.com It's only $19.50 and you'll not regret it.