Domain Name System is the system used on the Internet for mapping of names, such as www.google.com, to IP Addresses such as 18.104.22.168.
In many ways you can compare it to a phonebook. If you need to make a phone call to someone you can look up their name and then you are given a number that corresponds to that name.
In the same respect when you want to visit a website or send an email, you enter the easier to remember hostname, such as www.google.com, and not the harder to remember IP Address. The Domain Name System then takes this name, and converts it into the IP Address equivalent. This information is then sent back to your computer, and computer initiates the connection directly with that IP Address.