Devotees of Krishna decorate their bodies with markings called tilaka. Tilaka is generally made by mixing gopi chandan—a type of Indian clay—with water, and applied with the ring finger and/or with wooden stamps. The forehead marking is the most visible, but devotees place similar marks in thirteen places—on the arms, chest, neck, and back—while reciting mantras, prayers for Krishna's protection.
As devotees of sports teams often decorate themselves with their team's colors and insignias to remind themselves and others of the glories of their team, Krishna devotees wear tilaka as a reminder that we are all sparks of living spiritual energy within material bodies.
Tilaka is meant to designate the body as a temple of the spirit within, marking it as the property of the Supreme Person, Krishna, meant to be utilized in His service. Krishna Himself decorates His own face with tilaka; so anyone who sees the tilaka marking on the forehead of Krishna devotees is reminded of Krishna—either consciously or subconsciously.