This is the Unique symbol of the ONE UNIVERSAL CREATOR...GOD.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/discourses-in-english/8304-ek-omkar.html
The Word begins with OORRA..the FIRST Letter of the Gurmukhi Alphabet....signifying GOD comes FIRST...then the OORRAA is OPEN..and Extended wide without End...signifying GOD has no End..and takes in everything Under Him...and to further EMPHASISE the ONE...Guru Ji added the Numeral EK - ONE before the open oorraa...
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=8304
Thats as far as my limited understanding goes..others may wish to addtheir own comments..
Jarnail singh gyani