The tilak or pottu invokes a sense of sacredness in the wearer as well as others and is a religious mark. Its form and color differ according to one's status, religious group or the form of the Lord worshipped.
In ancient times, the four castes based on their varna or colour - Brahmana, Kshatriya, Vaishya and Sudra - applied marks in a different way. The brahmin puts on a white chandan mark suggesting purity, as his profession was of a priestly or academic nature. The kshatriya applied a red kumkum mark representing bravery as he belonged to warrior races. The vaishya sported a yellow kesar or turmeric mark representing prosperity as he is a businessman or trader keen on the creation of wealth. The sudra wore a black bhasma, kasturi or charcoal mark suggesting service as he supported the work of the other three classes.
Vishnu worshippers smear a chandan tilak of the shape of "U", Shiva worshippers a tripundra of bhasma, Devi worshippers a red dot of kumkum and so on.
Tilak covers the area between the eyebrows, which is the seat of memory and intellegience. In yogic language it is known as Aajna Chakra. The tilak is put on with a prayer - 'May I remember the Lord. May this pious feeling pervade all my activities. May I be righteous in my deeds'. Even when we forget this prayerful outlook, the mark on another person reminds us of our determination. The tilak is therefore a benediction of the Lord and a guard against the wrong tendencies and harmful forces.
The whole body emits energy in the form of electromagnetic waves - especially the forehead and spot between the eyebrows. That is why stress generates heat and results in a headache. The tilak and pottu cools the forehead, shields us and stops energy loss. Occasionally the entire forehead is smeared with chandan or bhasma. Making use of plastic reusable 'stick bindi' is not very helpful, even though it aids the purpose of beautification.