Tilaks are Special Marks on the Forehead

Some Hindus put a mark on their forehead; this mark is called a tilak. Different Hindus may use different markings. Some put a round red dot on their foreheads. Some Hindus mark their foreheads with three horizontal lines. Some mark their foreheads with a U shape and a dot in the middle.

We use symbols to help us think about things. For example, we may use a logo for our school, or a flag for our country. In the same way, we also need [these kind of] symbols to help us think about and link with God.

Reference: Primary Hinduism by Seeta Lakhani (Ch.1, 'What Is Hinduism', p.7-8)