Its a mark of auspiciousness.
It is put on forehead with sandle paste , or kumkum to activate the third eye chakra also.
According to some books,
in ancient world, tilak was also done by soil mother nature. You might not aware of the curing and healing capabilities of
soil. The application of soil to forehead signifies that it will pure the body and soul. Also , the tilak indicates straight
spinal a symbol of good health.
The devotees of shiva
apply ashes(bhasma) on the forehead.
The devotees of Vishnu
apply sandal paste (chandan) on forehead
The devotes of devi or
shakti apply kanku ( red turmeric powder)
A women without a tilak
is assumed to be not merried in hindu customs.