This symbol ੴ pronounced Ek Onkar is the symbol that is used to represents the “One Supreme Reality” or “One God.” This is the symbol that appears at the beginning of the Guru Granth Sahib Ji. Its the icon that starts the holy text which was first written by Guru Nanak Dev JI.
This fundamental teaching of Sikhism, is that there is only “one Essence” or “one reality” that sustains all; this is paramount to the understanding of Sikh beliefs.
Bhai Gurdas Ji says of Ek-Onkar:
By writing 1 (One) in the beginning, it has been shown that Ekankar, God, who subsumes all forms in Him is only one (as opposed to the two or three of Christianity or Siva, Bhrama and Vishnu or the thousands of Hinduism).
Ura, the first Gurmukhi letter, in the form of Oankar shows the world controlling power of that one Lord…