Festin literally translates to "feast". It's a beautiful word that evokes a sense of grandeur and celebration, much more than a simple repas (meal).
In this song from the movie Ratatouille, the word represents the ultimate dream of the main character. After a life of hunger and struggle ("affaibli par la faim" / weakened by hunger), the promise of a festin symbolizes hope, success, and the joyful beginning of a new life.
Camille starts this track in the gutter, cold and starving. The narrator opens by comparing lovers' dreams to wine that brings joy or grief, but romance quickly takes a backseat to an empty belly: "Affaibli par la faim je suis malheureux / Volant en chemin tout ce que je peux" (Weakened by hunger I am miserable / Stealing everything I can along the way). Life has been rough, full of skipped meals and petty theft, boiled down to the harsh lesson that "rien n'est gratuit dans la vie" (nothing in life is free). You can feel the fatigue when she sings "Un voleur solitaire, est triste à nourrir" (A lonely thief is sad to feed). It is a bleak, scrappy existence, built entirely on survival.
Then, the mood pivots completely into defiance and celebration. She refuses to accept that "la course aux étoiles, ça n'est pas pour moi" (the race to the stars isn't for me) and asks for the chance to take flight. In French, when she cheers "Nous allons enfin nous régaler" (We are finally going to treat ourselves / feast), she signals the end of hiding in the dark. The thief steps out to set the table for a brand new fate ("Je dresse la table de ma nouvelle vie"), declaring that the misery is over and the true banquet is right ahead.