The Domain Name System (DNS) is a hierarchical and decentralized naming system for computers, services, or other resources connected to the Internet or a private network. In other words a DNS server finds an IP address (example: 64.233.177.113) associated with a domain (example: https://google.com/).

Your Internet Service Provider supplies a DNS but they can be slow therefore finding a website take longer or not load at all. Public DNS servers may offer faster and may provide content filtering.
Some public DNS servers are:
- Google Public DNS, 8.8.8.8 and 8.8.4.4
- OpenDNS, 208.67.222.123 and 208.67.229.123
- Cloudflare, 1.1.1.1
How to configure DNS servers on a computer.
How to configure DNS servers on a router.