What are Plug-N-Play rc airplanes? Well, in short, they can be classed as a 'hybrid' category somewhere between RTF (Ready To Fly) and ARF (Almost Ready to Fly) airplanes.

Although the term plug and play is used as a very generic term for many different products these days, in this context the names 'Plug-N-Play' and 'PNP' are trademarks registered to Illinois-based rc product distributors Horizon Hobby.

So what does Plug-N-Play mean?... When you buy an RTF rc airplane everything comes with it in the purchase, including the radio control gear to fly it. When you buy an ARF rc airplane, you need to buy the radio gear and motor (, ESC & battery pack if electric) separately to install yourself.

A Plug-N-Play airplane has the motor, ESC if electric, and some of the radio gear installed but is missing the transmitter, receiver and motor battery pack (& charger). In other words, the airplane comes 99% assembled just like an RTF model but you need to supply your own transmitter, receiver and battery pack. 
And because most receiver slots and servo connectors are standardized with the 'Futaba/JR/Hitech' system, almost any radio gear can be used. If there are compatibility issues, they will be pointed out in the instruction manual and you'll more than likely be able to buy an adaptor lead somewhere.

Above, a Plug-N-Play plane is identical to an RTF one from the outside

Plug-N-Play rc airplanes are the perfect answer for modelers who want to buy and fly more than one RTF rc airplane, but don't want to have a separate transmitter for each one. This makes perfect sense of course but is only realistically possible if your transmitter has multi-model memory. Fortunately almost all modern Tx's do.