DNS is the phonebook of the Internet. It translates human readable domain names like apple.com to computer addresses such as 11.423.35.1. DNS is the first step of looking up a webpage.
Most of us use the DNS servers provided by our ISP. DNS in not encrypted and your ISP can easily see all your internet DNS requests. Once again, your browser's private or incognito mode does not anonymize your internet requests.
DNS queries are sent in clear text (using UDP or TCP)
ISP's May Share DNS
ISP can share your DNS queries with third-parties unknown to users
Embedded User Information
Some ISPs embed user information within DNS queries
Even using a VPN, leaked DNS queries can give away your online behavior