The requirement is that Sardinia-Piedmont/Italy owns the 3 provinces of Lombardia as well as the three Sardinia-Piedmontian provinces Nice, Chambéry and Annecy, and that Sardinia-Piedmont/Italy has a relation of at least 150 with France. Lombardia are initially owned by Austria.
It gives Italy/Sardinia-Piedmont cores on those three provinces, as well as removing 10 infamy and gives 20 prestige. On the other hand, it secedes the three provinces Nice, Chambéry and Annecy to France, replaces the Italian and Sardinian cores with French cores. It also improves relations with France by 50 effectively boosting it to maximum of 200.
It is the decision in the game, that remove the most infamy.