The Domain Name System (DNS) is used to translate domain names (like www.onecent.app) into numerical IP addresses (like 192.0.2.1) which are required to route packets of data on the Internet. Whenever your computer needs to contact a server on the Internet, such as when you enter a URL into your browser, your computer contacts a DNS server and requests the IP address.
When you connect to an anonymity network like a VPN, your operating system is supposed to use the DNS servers provided by the VPN network. However, in some configurations, the operating system may fall back to the default DNS servers (usually provided by your ISP). This means your ISP can see exactly which websites you are visiting, completely defeating the purpose of the VPN.