DNS propagation is the period between the change of DNS records for a DNS Zone of a domain and its propagation by the different DNS Resolvers globally, such as the DNS resolvers of your Internet service Provider (which usually are set as default on your service’s router) or public server resolvers, such as Google Public DNS, CloudFlare, OpenDNS, among others.
How DNS works?
Below you can see a video that tries to explain how the DNS system works: