DNS works like a phone book for the internet: it tells browsers and email servers where your website and email live. Your domain's records say, for example, which server shows your website and which server receives your email.
When you change a record or nameservers, the change spreads across the internet. It often works within an hour, but can take up to 48 hours to reach everyone. Until then some visitors see the old site and others the new one. This is normal.
