For all intents and purposes, DNS propagation means the movement of new DNS data to different internet providers around the world.
This DNS data does not travel to each of the Internet backbones in a straight line, it travels through streets and avenues, and can take up to 48 hours before it is updated everywhere. The importance of this DNS data is that it tells a users computer where to find your website, and therefore whilst it is still ‘propagating’ your computer may not know where your website so you may find yourself needing to wait longer to view it.
This will affect you if you’re changing the name servers on your domain, updating A or CNAME records on your DNS zone, or modifying any other information contained in your DNS zone. This means planning your DNS changes at low traffic times, or if you are familiar with TTL’s (time to live), setting those to a lower value. Propagation starts from the moment you make any adjustment to your DNS configuration.