Why do Hindus wear a Tilak on their forehead?

Shaivites (worshippers of Shiva), for example, smear their foreheads with three horizontal lines of vibhuti, a sacred white ash that acts as a reminder of the temporary nature of the material world. Made of the burnt dried wood from Hindu fire rituals, the three lines of vibhuti are called tripundra, and represent Shiva’s threefold powers of will, knowledge, and action. Tripudra is also frequently worn with a dot made of kumkum (a powdered red turmeric) in the center, symbolizing the creative and energetic force of the Divine known as the Goddess Shakti.
Vaishnavas (worshippers of Vishnu) usually apply tilak using sandalwood, clay, or a mixture of both, in two vertical lines, which connect near the bridge of the nose to form a “U” shape called urdhva pundra. Though the depictions and layers of symbolism of urdhva pundra vary depending on the sect of Vaishnavism a person belongs to, the “U” is commonly seen as the footprint of Vishnu, indicating the wearer’s desire to become his humble and devoted servant.

The word Shruti literally means “heard” and consists of what Hindus believe to be eternal truths akin to natural law. These texts are revered as “revealed” or divine in origin and are believed to contain the foundational truths of Hinduism.
The second category of scripture is Smriti, which literally means “memory,” and is distinguished from Shruti in terms of its origin. Teachings in Smriti texts are meant to be remind adherents the eternal truths of Shruti, and read and interpreted in light of changing circumstances over kala (time), desha (land), and guna (personality).