DNS gives your PC all the info on how to contact Domain Names like reddit.com or phtn.app. Your PC does that a lot and all the time. This connection is normally not verified or checked or encrypted. If you didn’t touch your DNS configuration you probably use your provider’s DNS server. So they know which services you use, which OS, where and when you go to which pages, and because there is no encryption anyone else may know or even change that, too. Instead of sending you directly to reddit.com a malicious entity might want to send you to another server first to do bad stuff. This is what makes it not secure.