Main Dining Room is included in the cruise fare so there is no additional cost to eat at this venue. Remember, beverages are not usually included and will be extra.
Main Dining Room is a "Sit Down" restaurant. This means you have waiter who will take your order and bring you your food. It usually takes a bit longer than quick serve / to-go establishments, but has a nicer atmosphere.
They're usually open for breakfast, lunch, and dinner. Specific times and schedules may vary, so always check in your cruise schedule or app to see the exact times.