The literal translation of the French phrase "confiserie des flandres ad pruvost lille" is "Flandres confectionery (or sweet shop) Pruvost Lille."
Provost is actually just a name, and Lille is a large city in the north of France.
Ballard's, a catalog company, is the producer and retailer of a line of decorative signs that bear this phrase, and they have created some confusion as the phrase doesn't translate easily and doesn't even have a very clear meaning.
For example, the "ad" part isn't even a French word... Although sometimes people shorten the word adresse (adress) to just ad, it seems a little strange that this might be the case here.
Anyway, to sum up, the closest translation points to the sign being a replica of a local sweet shop in Lille, France - perhaps owned by the Flandres family from the area of Provust?