DNS propagation is the time to wait after a change in the information of a domain name so that it is up to date on the Internet. It takes place after modification of DNS records or name servers.
Each time a domain name is used on a connected device (computer, smartphone, tablet, etc.), its DNS information is stored in the cache. This can be a local cache on the device's operating system, a DNS cache stored by the ISP, etc.
DNS records are stored in the cache primarily to improve the performance of DNS queries. Each DNS record has a TTL (Time to Live) value, which is the length of time that DNS servers must store that record in cache. When a record is modified, DNS servers will continue to use its formal value from cache until the end of this period.
In simple, DNS propagation is the time it takes for DNS servers around the world to update their cached information for a domain name. It is influenced by the lifespan of DNS records that may have changed, but other factors may also be involved.
A DNS change takes up to 72 hours to propagate around the world, although it most often happens in just a few hours.