Port groups aggregate multiple ports under a common configuration and provide a stable anchor point for virtual machines connecting to labeled networks.

vSphere Standard Switch Network
A vNetwork Standard Switch network

Each port group is identified by a network label, which is unique to the current host. Network labels are used to make virtual machine configuration portable across hosts. All port groups in a datacenter that are physically connected to the same network (in the sense that each can receive broadcasts from the others) are given the same label. Conversely, if two port groups cannot receive broadcasts from each other, they have distinct labels.

A VLAN ID, which restricts port group traffic to a logical Ethernet segment within the physical network, is optional. For a port group to reach port groups located on other VLANs, the VLAN ID must be set to 4095. If you use VLAN IDs, you must change the port group labels and VLAN IDs together so that the labels properly represent connectivity.