It is a combination of the five Olympic rings and the year. The design came from the architect and President of the Organising Committee for the Games, Pedro Ramírez Vázquez,  who lead the team that create all the icons, fonts and images for the Mexican Olympic Games.
Eduardo Terrazas (MEX) and Lance Wyman (USA) were part of that design team, that mainly recalls the patterns of the Huichole Indians.
IOC / Olympic Museum Collections
<a rel="nofollow" class="external free" href="http://www.olympic.org/uk/games/past/collector_uk.asp?type=5&amp;id=18&amp;OLGT=1&amp;OLGY=1968">http://www.olympic.org/uk/games/past/collector_uk.asp?type=5&amp;id=18&amp;OLGT=1&amp;OLGY=1968</a>