plaque de platre is French for plasterboard, so it's a little bit of a surprise to find this sold by the litre.
It could be anything I guess, but most likely:-
If it is very runny, it is a first coat primer for plasterboard before painting. In the UK you would just add 15% water to white emulsion for the first coat. This is because the plasterboard will take in the moisture too fast for normal paint. In France, emulsion paint is rubbish. It is not at all the same as in the UK. You can't water it down safely and also you must wait 24 hours between coats because until it is totally dry the colour is not set and a second coat after a few hours will be have no real effect (esp. in white paint).
If it is a little thicker than paint (like a runny paste) then this is a French equivalent of a skim coat, although this is not really used that much. Boards here tend to be mapped (glued) onto the wall and the joints done with a jointing compound. Then sanded and painted without a skim coat.