When 3D tracking, you have a pixel error. Is that error the number of pixels the track will stray with each frame, or the entire track?
The re-projection error is the difference in pixels between where the tracker is placing a point in the image, and where it is being re-projected in the geometry when the scene is Solved.
In the perfect world there should be no error, but in the real world there is usually some degree of discrepancy, given the nature of trackers, optics, and user error.
Usually tracking is considered accurate when the re-projection error is less than 1/3 of a pixel (0.3 pixels).
The number is always an average.
You can see the average per tracker (for however many frames the tracker is active) in the dopesheet
Or you get a report of the average of all bundled tracks for the whole scene at the bottom of the page.