A main node is a unified endpoint within a cluster which oversees activity on each node.
Main node responsibilities consist of scheduling deployment and distributing work to be installed on the nodes. Worker nodes that run the services can run or stop based on main node requests.
Worker nodes self-report statuses and activities to the main node which determines whether nodes need to be repaired, upgraded, or removed. The main node can add or remove nodes if a node fails.
Some projects refer to the main node as a master node. At DigitalOcean, we use main node in alignment with our considerate language guidelines.