Vermilion or red powder made of turmeric and lime worn by the Hindus at the point of the third eye on the forehead.
The important thing is that it is turmeric. Applying it has certain health and other benefits. Another thing is it is used as a symbolism in the society. If a woman is wearing kumkum, she is married – that means she is not approachable. This is just an indication, so that you do not have to tell everyone. In the western world, if someone is wearing a ring, you know they are married. Here, the symbolism was that if a woman is wearing toe rings and sindhoor, that means she is a married woman. She has other responsibilities. Socially, this is a way of organizing a situation where there is a clear marking, a social way of discerning who is who in the society.