It means two dinners for 2. So, 120 days prior to departure at 12 midnight eastern time, you log on to your NCL account and make two dinner reservations for 2 people. If you are going to a cover charge restaurant (e.g., Teppanyaki or Moderno Brazilian Steakhouse), book them first. Then book any a la carte restaurant.
You get 1 appetizer, 1 soup/salad, 1 entree, sides (if the restaurant has them), sauces (if the restaurant has them), and a dessert.
An example Free meal at Cagney's Steakhouse is