In-pillow headrest DVD players are built into the headrest pillow to allow for that factory install look as well as possibly hide the monitors from outside view or possible theft.
Clamp-on headrest DVD players do just as described. They clamp onto the metal posts of your existing headrest pillows to allow for an easy install and to not disrupt the "Active Headrest Technology" that may be included as a safety feature in your vehicle.
Active Headrest Technology is a technology where the headrest pillows will move during the event of an accident to help reduce the possibility of neck or head injury.
Pass-through AV technology is the ability to pass-through the audio/video signal from one unit to the next unit. For example the Autotain DREAM units have pass-through technology where you can watch your in-dash unit on DREAM headrest 1 and it will pass-through to DREAM headrest 2 so you can watch the in-dash on both headrest units.
Headrest screen resolution is much the same as for TV screens. The resolution is measured by the number of pixels across and down the screen. The more pixels, the higher the resolution and the better quality the image.