Nodes may also be known as 'slaves'. In essence a node is a machine or processor that can be used to process the render. Some applications refer to an entire machine or its processor as a node, whilst others may classify any processing thread as a node (modern processors often incorporate multi-threaded, multiple cores). V-Ray and Maxwell Render consider a machine to be the node.
By joining up render nodes on a network in this way, you are creating a render farm.
Maxwell Render - Render Node Setup