Ik Onkar is the symbol that represents the One Supreme Reality and is a central tenet of Sikh religious philosophy. Ik means one or united, On means supreme, ultimate, or highest bhrama (God), and the Atma (Soul) of the entire universe or system, and kar means without shape or form.

It is a symbol of the unity of God in Sikhism, and is found on all religious scriptures and places such as Gurdwaras. Derived from Punjabi, Ik Onkar is the first phrase in the Mul Mantar referring to the existence of “one constant”

It is found in the Gurmukhi script and is consequently also part of the Sikh morning prayer, Japji Sahib.

source: https://en.wikipedia.org/wiki/Ik_Onkar