It means beryllium diffused. Basically the structure of the crystal is changed by introducing Beryllium during the heating process (I think it was discovered by heating Beryl at the same time as Corundum). Gem stones are very often subjected to various types of treatment to 'enhance the colour' therefore (sometimes this enhancement is only 'skin deep') increase there value. In a perfect world you want to buy an untreated stone. Heat treatment is very common and can improve colour and clarity - basically finishes the job nature started. The Germans used to be renowned for their exporting of high quality blue Sapphires . The Thai's do it better.
At the other end of the scale you have treatments like Beryllium diffusion (Titanium diffusion is another). It changes the stone into something it could never be - Not natural.
The trouble is, this sort of treatment is all to common and many unsuspecting buyers think they have a bargain. (So what you could be buying is what started off as a ugly pebble has been manipulated to look like an impressive gem stone)
And it's all the way down to your cheaper stones. IIRC generally most Citrine is mainly heated Amethyst. The Beijing Olympic games stone - yet another(only found that one today).
HTH
Mr Magoo still off topic.