From http://hugin.sourceforge.net/docs/manual/Hugin_Control_Points_table.html:

Distance, the distance in pixels between a perfect alignment and the actual alignment achieved by the optimiser. Otherwise, after selecting Fine-tune all Points from the Edit menu, this column shows the correlation between the points (0.0 indicates no correlation and 1.00 indicates 100% correlation) - Typically values over 0.8 show that the image areas around each point of the pair are very similar (an 80% correlation).

also (emphasis is mine):

Clicking Select by Distance allows you to select all the points depending on the value in the Distance column, eg:

• Enter 10 to select all the points with an optimiser error greater than ten pixels.
• Enter -5 to select all points with an optimiser error less than five pixels.
• Enter -0.8 to select all points with a fine-tune correlation less than 80%.

When Hugin gives you a number greater than 1 as distance, it is indeed expressed in pixels. When between 0 and 1, it is a correlation value.

Still, exactly what Hugin means by the distance in pixels between a perfect alignment and the actual alignment achieved by the optimiser isn't very clear, but it's probably processed from actual pixels distance between the real points and their expected corrected positions.