DNS servers play a crucial role in modern networking. When you type in a website, like androidpolice.com, your DNS looks up the IP address tied to that domain so the page can be loaded. Most people stick with the default DNS from their internet service provider, but because ISPs have been known to log DNS queries, some turn to alternatives from Google and OpenDNS.
CloudFlare released its own public DNS service last year, which supports DNS-over-TLS/DNS-over-HTTPS and is faster than Google's service in some cases. CloudFlare has just released an Android app that makes the process easier.
The 184.108.40.206 app re-routes your device's DNS requests through CloudFlare's own servers, but it uses a local VPN server to do so. This means you can't use other VPNs at the same time. Android 9 Pie allows you to change your system-wide DNS easily, but if you're on an older version of Android (like 99% of the global population), this is a nice alternative.
You can download the app from the Play Store below or from APKMirror.