What is a distributed switch?

Distributed port groups are used to connect virtual machine ports.
Standard switch and distributed switch
It can be exchanged across hosts, and can always communicate without changing the port group when a virtual machine is migrated.
Acts as a single switch between all associated hosts. This allows virtual machines to ensure consistent network configuration when migrating across multiple hosts.
Distributed switches can forward internal traffic between virtual machines or link to an external network by connecting to a physical Ethernet adapter (also known as an uplink adapter).
Each distributed switch can also be assigned one or more distributed port groups. Each distributed port group is identified by a network label that is unique to the current data center.
The VLAN ID is optional and is used to restrict port group traffic to one logical Ethernet segment within the physical network.