Propagation is the period of time, or delay, involved in sending your domain's address information to all the various name servers in the world. Name servers intentionally keep track of addresses for domains in their memory for a specific period of time (defined by the administrator of the name server). This speeds up the process of looking up an address for a domain name. Unfortunately this "cached" information also stays in the name servers when it has been changed at the source of the original information (the domain's registrar).
Name servers refresh themselves from once an hour to once a day. Experience dictates that 2-3 days is a good estimate for the period of time from when a domain's DNS information is changed at the domain registrar to when everyone in the world can see the change.