HBase is designed on top of HDFS and thus has a Master-Slave architecture same as underlying HDFS.
The HMaster is responsible for Load balancing ,for moving regions from one region-server to another whenever required ,assigning regions on start up and many more operations .

Although the client directly interacts with the HRegionServer for Data but with few thousands node cluster having Single HMaster can be a Single Point of Failure ,one can also create the HMaster cluster by having HMaster in multiple nodes but at any single point in time only 1 HMaster will be active leading to bottle neck.