Propagation is the delay or period of time taken for your domain's address information to be updated in all the various Domain Name Servers across the internet.
DNS servers only keep the records of domain names and associated IP addresses for a specific period of time. The information is temporarily stored or "cached" and is updated anywhere from once an hour to once a day to take account of any changes and to remain as accurate as possible.
For a newly registered domain name this propagation can take anything up to 72 hours until all DNS records are updated across the internet.
