SAMP stands for Simple Application Messaging Protocol. It is a protocol which allows tools on the desktop to communicate with each other. Very briefly, the way it works is that applications can send messages to a central hub process which will then pass them on to other applications which can respond to them in some appropriate way. GAIA can use SAMP to send and receive images, sky position information, and catalogues or identification of rows within them. It can also send extracted spectra. You will require a hub, either free-standing or integral to some other application, to use these capabilities.
You can find more information about how it works, what applications are compatible, and how it can be useful at the SAMP web page. To remove the Interop menu you can use the Startup options... window located in the File menu.
SAMP is an evolution of the earlier similar protocol
PLASTIC support, which was provided in earlier versions of GAIA,
has now been withdrawn in favour of SAMP.
GAIA -- Graphical Astronomy and Image Analysis Tool