Learn Danish: Tak for mad! - Thank you for the meal!

In Danish, this is a fixed expression. Even though we translate it into English as "Thank you for the meal", the literal Danish is just "Thanks for food." You do not use the definite ending -en here.

When exactly do I say this in Denmark?

You say Tak for mad at the end of a meal to politely thank whoever cooked or provided the food. It is a very strong cultural rule to say this when you finish eating, especially when visiting someone's home.

How should the host reply when I say this?

The standard response from the host or cook is Velbekomme, which loosely means "You're welcome" or "I hope it agrees with you."

Can I use this phrase in a restaurant?

Yes, you can say it to the waiter when they clear your plates, though it is slightly less mandatory than when you are a guest in a private home. It is still a very polite and common thing to do.