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: 220.127.116.11) 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: