It's where your existing advance seat selection, being a nice BAEC status perk, becomes invalidated when the cabin divider is moved up or down the cabin on a short-haul aircraft, to change the split between Club Europe and Euro Traveller cabins.

The most common form of "curtaining" is where the Club Europe cabin is expanded, moving the curtain further back, and evicting passengers out. This can cause you with your nice preferred window/aisle seat in a halfway decent B/E Pinnacle, maybe even a spacious exit row seat, to suddenly be lumbered with a less than stellar seat (for instance the "ironing board" space-saver seats on the Neo fleet), especially if you are curtained very late in the day, and there's only middle seats at the back of the plane left.

But, it is also possible to be curtained in Club Europe if the cabin size is reduced to allow more Economy seats. Okay, you can't end up with a middle, you'll always get a window or aisle, and it will be a Pinnacle, but if you're travelling with a companion, your group might get split.

The danger zone for being curtained is usually between rows 4 and 10 on the 319, 4 and 12 on the 320, and 4 and 14 on the 321 fleet at Heathrow. Gatwick fleet differs.