An arbiter node doesn't have any dataset, hence it doesn't accept read/write operations. The arbiter member votes in the replica set election process for a secondary node to become primary. An arbiter node comes useful when you have an even number of replica set members or not enough servers for a new member.
Note
Add an arbiter node to a replica set only when there is an even number of members. If you add an arbiter node to a set with an odd number of members, the set might suffer from tied elections. For more information, you can visit http://docs.mongodb.org/manual/core/replica-set-arbiter/.
The following diagram shows you a replica set with an even number of members:
The preceding diagram enables a replica set to have an odd number of votes and prevent the replica set from the tied election problem.