Tilak is a special symbol that the Vaishnava people wear on their forehead. It's made up of two parts: the big circle in the middle and the smaller lines coming out of it.
The big circle represents the eye of the god Vishnu, who the Vaishnavas worship. It's like a special sign that shows everyone that the Vaishnavas love and respect Vishnu very much.
The lines coming out of the circle are called U-shaped marks. There are normally two or three of them, and they represent different things.
The first U-shaped mark represents the god Narayana, who is another form of Vishnu. The second U-shaped mark represents the goddess Lakshmi, who is the wife of Vishnu. The third U-shaped mark, if there is one, represents the sage Markandeya, who was blessed by Vishnu.
Overall, the tilak is a way for the Vaishnavas to show their faith and love for their god Vishnu, and to identify themselves as members of the Vaishnava community.