It is a small, green, metal object about the size of a .22 bullet. It is a radio device that Montag will put into his ear. It allows Faber to hear everything Montag hears and it also allows Faber to speak to Montag. It is essentially a smaller version of our modern bluetooth earpiece, except that it is green and small enough to fit neatly in the ear, and is therefore unseen by others.
Faber insists that he's created this device out of cowardice. With this hearing device in the world, Faber is able to sit comfortably at home, away from any danger. Montag places the green bullet in his ear and Faber places another in his own ear. They are able to communicate back and forth with these devices. Faber claims that he has been working with radio technology and electronics while hiding in his home. Faber adds that he has been designing devices such as the green bullet for a time such as this. Faber is reluctant at first but gives Montag the green bullet. Faber is finally going to put one of his devices to revolutionary use. Therefore, he is terrified and excited.
Given that this device is used for interpersonal communication in order to foment a revolutionary movement, there is some interesting similarity between this green bullet and our modern social media. Both can be used to connect people of like minds and interests.