Warning: this is quite a mundane explanation, not the exotic thing that many readers make out of glass candles
HOW THEY WORK
I hypothesize (for years now) that the glass candle is a type of magical psy-technology that allows for remote viewing and projecting in the present time, sort of like a live camera that allows for a hollogram of a messenger.
Some magic allows to look back into the far past. This is what a greenseer can do such as Bloodraven, Bran and some of the children of