Propagation is the process by which name servers are made aware of the changes that you make to your domain name with your registrar (the company you purchased your domain name with). This process can take between 24 and 72 hours.

Why does this process take so long?

DNS propagation takes a bit of time because the "synchronization" is happening amongst hundreds of thousands of servers. The process may seem to take too long but it is happening quite rapidly on a pretty large scale.