In HDFS, data is stored in blocks on a DataNode. There can be a situation when data is not uniformly spread into blocks on a DataNode. When we add a new DataNode to a cluster, we can face such a situation.
In such a case, HDFS provides a useful tool Balancer to analyze the placement of blocks on a DataNode. Some people call it as Rebalancer also. This is an administrative tool used by admin staff. We can use this tool to spread the blocks in a uniform manner on a DataNode.