Short answer
With a DNS leak, your internet provider or another DNS operator may see which domains your device looks up.
DNS is the address book of the internet
When you type a domain, your device needs to find the IP address behind that domain. This lookup is done with DNS.
Why it matters with VPN
If web traffic goes through the VPN but DNS queries go to your provider’s DNS, the provider can still see which domains are being looked up.
Protection against DNS leaks
Choose a VPN with DNS leak protection, use encrypted DNS where appropriate and test after major changes to VPN, router or operating system.
Common questions
Is DNS the same as IP?
No. DNS translates names into IP addresses. The IP address is then used for the actual connection.
Does private browsing stop DNS leaks?
No. Private browsing clears local history but normally does not change how DNS is sent.