It means that the children have mother-tongue or near-native competence in both English and French. The programme follows the normal French secondary school curriculum, plus additional hours for language and literature as well as history and geography taught in English. Children must be capable of speaking, reading, and writing in both English and French at a level expected of their peers in the UK and France. If you are not sure, ask yourself if your child would be capable of following an English or History class in the same year group in the UK or France.
For the 6ème test, allowances are made for the fact that many of the children entering will have come from a French primary school and focused day-to-day writing practice in English will not be part of their school schedule. (They are understanding about spelling.)