Microsoft Knowledge Base Article 262458 contains the following summary:
Universal Plug and Play (UPnP) is an architecture in Microsoft Windows Millennium Edition (Me) that supports peer-to-peer Plug and Play functionality for network devices. The UPnP specification is designed to simplify device and network service installation and management. UPnP accomplishes device and service discovery and control through a driver-less, standards-based protocol mechanisms. Universal Plug and Play devices can auto-configure network addressing, announce their presence on a network subnet, and enable the exchange of descriptions device and service descriptions. A Windows Me computer can act as a UPnP control point to discover and control the devices through a web or application interface.
You can install Universal Plug and Play support in Windows Me by using Add and Remove Programs in Control Panel. When Universal Plug and Play support is installed, and a Universal Plug and Play device is added to the network, the Windows-based computer acts as a control point for that device, and notification is provided on the taskbar that a new device is available.