What does Socket.IO abbreviation io stand for?

I'm new to socket.IO and wondering what some abbreviation stand for. What does the IO inside the code stand for? What is actually stored in it? What is io.sockets storing?

io.sockets.on('connection', function (socket) {

socket.on('message', function(message){

    handler[message.request](message.text, socket, io);
});


socket.on('disconnect', function(){

    handler['disconnect'](socket, io);

});
});