The chandlo (dot) worn on the forehead is a sign that one is a Hindu. It is called the bindi in the Hindi language. In olden days, all Hindu men and women wore these marks. When worn by men it is usually called the tilak/chandlo.
Lord Shree Swaminarayan has commanded that all His followers should have a Tilak Chandlo on their forehead. It should be done in four places and these are Forehead, Heart, Right Arm and Left Arm.
The vertical lines pointing upwards show that the Tilak mark leads upward to Akshardham (the divine abode of Lord Shree Swaminarayan). The red mark is the symbol of Laxmiji (spiritual prosperity) living in the heart of Shree Hari.
By applying the Tilak Chandlo, it protects you from worldly bondage and leads to external spiritual liberation. The body remains as holy as the temple of God. It burns off the mountains of sins.
The chandlo has a mystical meaning. It represents the third eye of spiritual sight, which sees things the physical eyes cannot see. Hindus seek to awaken their inner sight through yoga. The forehead dot is a reminder to use this spiritual vision to perceive and better understand life's inner workings, to see things not just physically, but with the 2mind's eye" as well.
There are many types of forehead marks other than the simple dot. Each mark represents a particular sect or denomination of the Hindu religion.
The Tilak is of more than one colour although normally it is vermilion. It also does not have any standard shape and form and is applied differently by members of different Hindu sects and sub-sects. It is applied as a 'U' by worshippers of Lord Vishnu and is red, yellow or saffron in colour. It is made up of red ochre powder and sandalwood paste. Worshippers of Lord Shiva apply it as three horizontal lines and it consists of ash. Soot (Abhira) is also used as a pigment for applying a Tilak.
For Hindu women the red bindi is generally a sign of marriage.