HMaster’s primary responsibility is to perform administrative operations on the HBase cluster.

A region server can serve one or more regions. Each region is assigned to a region server on startup and the master can decide to move a region from one region server to another as the result of a load-balance operation. The master also handles region server failures by assigning the region to another region server.

The mapping of regions and region servers is kept in a system table called META. By reading META, the HBase client API can identify which region is responsible for the key being searched for. This means that for read and write operations, the master is not involved at all and clients can go directly to the Region Server responsible to serve the requested data.

The following is the high level architecture of HBase copied from Cloudera's Engineering Blog:

In order to have High-Availability, HBase can be started with multiple masters in an active/passive manner.