Win, draw, loss probabiltes based on Elo

[luckis11]

Using past results and Harkness ratings, Elo observed that the distribution of individual performances resembles a normal distribution"

http://www.chessbase.com/newsprint.asp?newsid=4326

What exactly is this "performance"?

 

[mathman]

I would guess that it is a way of assigning a numerical value to the outcome of a player's performance in a tournament.