Propagation is the process whereby the nameservers throughout the world have updated their records for a specific domain. For example, if you move your domain from one host to another, it will take around 24 hours or so for the new address to broadcast everywhere. During those 24 hours, the traffic is decreasing at the old location and increasing at the new location. More information on this can be found at http://www.dynadot.com/community/article/qa.html?aid=16.