It means in clock-wise fashion and is used when our network consists of more than one DNS servers and we want request to resolve DNS query to be distributed to all DNS servers equally.
DNS server provides us load-balancing and all servers will get equal amount of traffic.
For example, if we have 4 DNS Servers and Round Robin is enabled. All servers having same contents, replicating with each other and four different entries with common host name but configured with four different IP addresses.
Name IP Address
www.mydomain.com 192.168.0.2
www.mydomain.com 192.168.0.3
www.mydomain.com 192.168.0.4
Now suppose, we have 4 different clients and each client enter the above URL on their browser. First request will be resolved by the first IP address, 192.168.0.1, then 192.168.0.2 and so on.
