The lightning bolt represented Elvis' "emblem", "Taking Care of Business, In a Flash". "Taking Care of Business", or TCB for short, was meant to be a symbol of the people who followed Elvis. He created this emblem after having met President Richard Nixon in 1970, and after receiving an Outstanding Young Men's Award from the Junior Chamber of Commerce (the Jaycees) in 1971.
Later, he penned a philosophy behind the lightning bolt which included "body conditioning", "respect for fellow man", and "freedom from constipation". (He never took himself too seriously.)