In Sikhism, the turban is a symbol of total equality.
Before Sikhism was the religion it is today, the turban was worn only by the wealthy upper classes. The person who started the Sikh religion donned the turban to show people that everyone is truly wealthy in spirit, and that it is the duty of all people, rich or poor, to help one another.
So the Sikh turban became a symbol of equality in humanity, and as a sign that that person can be turned to for help.