When bearing off checkers, you may notice a display showing the current epc value (next to the display of Pips). The epc display can be turned on or off under menu Settings —> Options —> Display (at least in the current version, as of June 2005).
epc stands for Effective Pip Count, and is an alternative measurement of the goodness of the position. The usual pip count can be a misleading goodness estimate when both players are bearing off. What is more important (for making cubing decisions) is the expected number of rolls each player needs to bear off hes checkers.
The effective pip count is defined as this expected number of remaining rolls, multiplied with the average number of pips in a roll, which is 8.167. (The average number of pips in a roll would be 7 if rolling a double wouldn't yield a double pip count – i.e. a double-6 yields 24 pips, not 12.)
Thus, epc takes into account not only the current pip count, but also the expected pip wastage:
It's clear (hopefully) that the expected pip wastage (and hence the epc) increases with more checkers on lower points (e.g. you get pip wastage when rolling a 6 to bear off a checker from the 2- or 1-point).
It is further explained on BackGammon Galore, especially in a post by Douglas Zare.
How do one calculate the epc then, or the expected number of remaining rolls? Gnu BG uses the one-sided bearoff database, which contains the change of bearing off the final checker in any given number of rolls. A player would probably use some quite complicated heuristics (please see the aforementioned post by Douglas Zare, and do get back here and explain it!)