DNS propagation
In one line
DNS propagation is the period after you change a DNS record when some people still see the old answer. It happens because resolvers around the internet keep a cached copy until its time to live runs out.
Nothing is travelling
The word propagation suggests your change is spreading slowly across the world. That is not quite what happens. When you save a record, your DNS host knows the new answer straight away. The delay comes from caches.
Your internet provider, your phone's network and many companies run resolvers that remember answers so they do not have to ask again every time. Each answer comes with a TTL, a time to live, which says how long it may be remembered. Until that timer runs out, a resolver keeps handing out the old answer.
How long it takes
Most changes show for most people within minutes to a few hours. The common advice to allow up to two days covers slow caches and changes of nameservers, which are cached at a higher level and for longer.
If you know a change is coming, you can lower the TTL on the record a day ahead. Then caches forget the old answer sooner when you make the switch. Raise it again once everything works.
Why you and a friend see different things
You might see the new site while a friend still sees the old one. That is normal during propagation: you are asking different resolvers, and each holds its own cached copy. Your own browser can cache too, so try a private window or another device before deciding something is wrong.
Checking progress
The DNS checker asks DNS directly and tells you in plain words what a name points at now. If it shows the new value, the change is saved correctly and the rest is waiting. If it shows the old value long after the TTL has passed, check that you edited the records at the nameservers your domain actually uses. The custom domain guide covers this for Mini Casa, where custom domains are in preview and the connection screen shows its status. More on that on the custom domains page.
Questions people ask
Can I speed up DNS propagation?
Not after the change, because caches keep the old answer until its TTL runs out. Before a planned change you can lower the TTL, so caches refresh sooner when you switch.
Why does the DNS checker show the new value but my browser shows the old site?
Your device or browser may still hold a cached answer or a cached page. Try a private window, another device or your phone's mobile data. Most of these clear on their own within the TTL.