Page "Flexible single master operation" Paragraph 4

* The Relative ID Master-Every domain in your forest has exactly one domain controller holding the RID Master role.

The purpose of this role is to replenish the pool of unused relative IDs ( RIDs ) for the domain and prevent this pool from becoming exhausted.

RIDs are used up whenever you create a new security principle ( user or computer account ) because the SID for the new security principle is constructed by combining the domain SID with a unique RID taken from the pool.

So if you run out of RIDS, you won't be able to create any new user or computer accounts, and to prevent this from happening the RID Master monitors the RID pool and generates new RIDs to replenish it when it falls beneath a certain level.