i think this link will help u very much
**broken link removed**
To explain round robin arbiter, its better to go through the priority arbiter as well.
Assuming there are 3 masters, A,B and C, with 3 different priority levels, high, medium and low. During initialization of the system, assuming A is given the highest priority, B is given medium priority and C the low priority. When there is a request by all the masters to access the bus, A will be serviced first. Once its serviced, the priority of A changes from high to low, B changes from medium to high and C changes from low to medium.
This remains the status quo till B is serviced and then it loses its high priority status and changes to low priority. This forms a cycle and this type of arbitration policy is called round robin arbitration. Hope i've been clear. Let me know incase of any clarifications or corrections