Sa7tein literally, means two healths and you will hear this said whenever and wherever food is being enjoyed. Your hosts may say it to you before beginning the meal and you can also say it to someone in the middle of eating when you approach them. The response is 3a 2albak!
A more subtle usage is when a friend or coworker is eating and you are around. To be polite they will say tfaDDalu and offer you some of their lunch. You can respond back politely by saying Sa7tein, telling them thanks for the offer.