A DNS server translates a domain name such as meon.zero.eu.org to an IP address such as 195.137.116.79
This is equivalent to translating: Natural History Museum, Cromwell Road, London to SW7 5BD
or
National Institutes of Health, 9000 Rockville Pike, Bethesda, Maryland to 20892
Unfortunately some people have preferences with DNS servers which invoke advertisements. These could be caused by a virus or by somebody supplying modem/routers with them. Several web pages claim that one of the UK's largest ISPs did this.
The simple fix is to use the DNS server I gave you.
Many modem/routers are located at 192.168.1.1
All addresses starting 192.168 are on your private LAN.
You could access your router and change its DNS server address which it passes on as the default to computers connected to it.