A balconette bra is a type of demi cup bra, which basically means the cups are low to allow you to wear fairly low cut clothing. A balconette bra usually has a more of a horizontal line along the top of the cups, which makes it quite different from a plunge bra, which goes very low between the breasts.
‘Balconette’ has become a loosely used term, so when you see it you can’t really be sure what you are getting. Some people even use it to describe a shelf bra (cupless) bra. The one thing all ‘balconette’ bras should have in common is that they have quite low cut cups, underwires and quite widely set straps. In saying that I have seen bras described as ‘balconette’ when they are almost full cup. Since the word sounds good in marketing it seems to have been overused to the point where the meaning has mostly been lost.