When you change the default DNS servers you're changing the servers assigned by your ISP that your device uses to convert hostnames to IP addresses.

Changing DNS servers can be useful for certain Internet connection problems. It might help keep your web surfing more secure and private and could even allow you to access websites blocked by your ISP. Also it may speed up your internet connection; some users have seen improve in online gaming (lower ping) when changing DNS servers.

Fortunately there are many public DNS servers that you can use instead of the default ones. This app will allow you to choose from a list of free and reliable public DNS servers (primary and secondary DNS servers) like Google Public DNS OpenDNS etc.