When you visit sites, usually you will search domain names likewww.google.com. But it can be recognized only when it is converted to IP like 10.255.255.255.
DNS is a 'web address book lookup system' where your device uses a website address in IP address, such aswww.google.com to 10.255.255.255.
Normally, your device will send a DNS request to your ISP(Internet service provider) DNS server when accessing the internet.
However, while using a VPN service, your device should only send DNS requests to your VPN service provider's DNS server. If your DNS request is still sent to your ISP DNS server while you are using a VPN, then your VPN provider is leaking your internet traffic.
You can see if there is a DNS leak test inside X-VPN