Aum or Om is a Mandhram (Mantra) and the most sacred symbol of Hinduism. Aum is said to be the essence of all mantras, the highest of all mantras or divine word .
A stands for Creation, U stands for Preservation, M stands for Destruction or dissolution. This is representative of the Trinity of God in Hindu dharma (Brahma, Vishnu and Shiva).

Found the Symbol at the wall of Srinivas Youth Sangh near the Triplicane Sri Parthasarathy Temple.