DNS for a domain means to what IP address they translate to. When you change what IP address your domain points to, it takes time to update those DNS records to all the DNS servers throughout the world.
The update of these records are referred to as propagation
DNS Propagation is a process by which all of the DNS servers around the world collect updated information regarding the location of your website and how to find it. The time it takes for DNS record changes to update on end-user's systems widely depends on the settings of the actual end-user's DNS server, but can range anywhere from 4 to 72 hours.
