The Domain Name System is the system used on the Internet for the mapping of names, such as www.google.com, to IP Addresses such as 216.239.51.99. Every time you connect to a site, your ISP DNS Server must find out what name server has the information about the domain for the site you are trying to reach. If your ISP’s name server is not working or can’t be reached, you will not be able to traverse the Internet using hostnames, but instead would have to use their IP Address equivalent.
Source: https://www.bleepingcomputer.com/tutorials/the-domain-name-system/