Each domain has at least two nameservers assigned to it that hold the domain's records.
When a server worldwide queries the nameservers for DNS records, it will most often take a local copy of the DNS record. This means future lookups can use the cached answer rather than querying the original nameservers again.
After modifications reach the original two nameservers, the changes gradually distribute across global servers until full synchronisation occurs. It is this that can take 24–48 hours and is known as propagation.