A DNS server is what converts a domain name, like google.com, into a numeric IP address, which is required for Internet traffic. Think of it like the mail room in an office building. They get mail for Bob in accounting, but the mail only has the name and the building's address. The mail room staff (DNS) knows what floor and desk Bob sits at.

Since many ads are hosted on their own domains, like doubleclick.net, you can block them at the DNS level so your device never actually connects to the ad server.

By default you're probably using your ISP's DNS server, but you can customize it.