Sambar is referring to a South Indian vegetable curry that is made largely from lentils. It is a famous and traditional South Indian stew made mainlywith vegetables (eggplant, squash etc) cooked with crushed lentils, onions, chillies, Coriander and Turmeric and with a pinch of Asafoetida.
Note: I was blessed enough to enjoy this dish during my stay in South India, particularly in Tamil Nadu and Kerala. It was even prepared tradionally and authentically by Kerala's housewives so it was much, much better than what is served iin restaurants.
List of books: Sambar