About DNS propagation
When you change a DNS record, the update doesn't reach every resolver instantly, each caches answers for the record's TTL (time-to-live). During that window, different users may get different answers depending on which resolver they're using. This tool queries a spread of well-known public resolvers in one shot so you can see at a glance whether the new values are spreading correctly. If results are inconsistent, wait for the old TTL to expire before declaring the rollout complete.