eckhard's post explains it, but just to flesh it out a little more:
FD refers to Flight director, obviously.
The grouping of the letters and numbers show which flight director is engaged on which PFD. So the number on the left side of the 'FD' letters shows which flight director is displayed on the left PFD, and the number on the right side of the 'FD' letters show which flight director is displayed on the right PFD.
Normally one sees 1FD2, which means flight director one is displayed on the left PFD and flight director two is displayed on the right PFD.
If there are any FMGC failures, the status of the flight directors can be instantly seen:
2FD2 : Flight director number two is being displayed on both the left and right PFD's (i.e., both PFD's show the same flight director: there is no independant cross check.)
-FD2 : The left flight director is not selected, and flight director two is selected on the right PFD.
1FD- : Number one flight director is displayed on the left PFD, the right PFD has no flight director selected.
1FD1 : Flight director number one is displayed on both PFD's.
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