What is the DDC PnP port?

Port 1 is labeled "DDC PnP Port" because that is the port the splitter will read the DDC signal and/or Plug-n-Play Compatibility from. The other ports take their cues from Port 1.

If you plug a DDC compatible monitor into Port 1, all other monitors must be able to support the highest resolution of the monitor connected to Port 1.

DDC stands for Display Data Channel, and is a handshake standard that enables a video monitor to relate information (such as maximum resolution and depth) to a video card source. This allows the source and display to synchronize themselves for optimal display settings.