The new tool is a Chrome extension. You may remember Chrome extensions were launched into Beta yesterday - but they're not yet available for Mac users.
Speed Tracer is designed to help web developers recognize slow areas in their applications. The tool can be found in Google's Web Toolkit.
It displays performance along a "Sluggishness Graph." This is one of those things where the lower the score the better. If your y-axis is closer to 0 - you've got a fast app/site. If it's closer to 100 - keep working, buddy.
Here's a video Google put together to provide a visual tour of Speed Tracer.