DNS stands for Domain Name System. DNS server is a key part of the web, their job is to translate the human friendly domain addresses (example: portchecker.co) into the machine friendly IP address (example: 45.33.50.110) so that your device can communicate with the server and get things done.
Sometimes, the default DNS server from ISP may be slow or sometimes they also block certain sites at DNS level or for some users they may not be secure enough. In either of the case, you're better off using one of the public DNS servers listed below.
List of free public DNS serversServers are listed with their IPv4 and IPv6 addresses.
DNS ProviderPrimary DNSSecondary DNSGoogle - IPv48.8.8.88.8.4.4Google - IPv62001:4860:4860::88882001:4860:4860::8844Quad9 - IPv49.9.9.9149.112.112.112Quad9 - IPv62620:fe::fe2620:fe::9OpenDNS - IPv4208.67.222.222208.67.220.220OpenDNS - IPv62620:119:35::352620:119:53::53OpenDNS - FamilyShield208.67.222.123208.67.220.123Cloudflare - IPv41.1.1.11.0.0.1Cloudflare - IPv62606:4700:4700::11112606:4700:4700::1001Verisign64.6.64.664.6.65.6CleanBrowsing - Family Filter185.228.168.168185.228.169.168CleanBrowsing - Adult Filter185.228.168.10185.228.169.11CleanBrowsing - Security Filter185.228.168.9185.228.169.9Using a DNS server from the list above will reduce your DNS lookup time and it may speed up your browsing experience. If the default DNS server is slow or causing any issue or you want extra security features, it's worth a try. Public DNS servers could protect you from malicious and phishing sites (primarily designed to steal your sensitive data) or inappropriate/adult sites that you may want to block for kids / family users.
Note: You could also ping to the DNS servers and select the one based on the latency (lower the ping the better it is) to keep the DNS resolving time as low as possible.