Round Robin DNS is a technique where multiple records with the same host entry but a different destination exist.
The DNS server will serve all the records when queried, but every time in a different order. For a three record scenario this would be as follows:
A B C
B C A
C A B
The iterations in the order of the records is why the technique is called Round Robin.
Most clients pick the first records served, this causes clients to use a different destination every time a query is done. Resulting in a way of load distribution.
Do note that depending on the service this technique is used for and the way this service is configured, it does not always result in a ideal loadbalancing situation.
For example when using this for email services a client will try a given MX record “A”. If the service does not respond the client will try a second given MX record “B” and so on. In the case of email services the client will just try all MX records available taking care of a possible server fauilure.
If however a http service is loadbalanced using Round Robin DNS, the web browser will be served A record “A” and try to connect to that. In case this service is not available the web browser will not try a second or later A record but display a server failure.
See for detailed information the RFC: https://tools.ietf.org/html/rfc1794
