The ROS Master is the main component of the distributed ROS system. It is responsible for maintaining the network and keeping track of the nodes and topics in the system. The ROS Master also registers and coordinates nodes, topics, services, and parameters.
As an example, when a node is launched on a ROS system, the ROS Master will register it and keep track of its topics, services, and parameters. When a node publishes a topic, the ROS Master will ensure that the topic is available to all other nodes in the system. Similarly, when a node requests a service, the ROS Master will ensure that the request is routed to the appropriate node.