I assume that you are referring to the device that Faber gives Montag to put in his ear.
The device (referred to as "the bullet") is about the size and shape of a .22 caliber bullet. It fits in the ear and people will not necessarily know it's there. It is a two-way communications device. Faber presumably has some sort of device in his house that communicates with the bullet. It lets him speak to Montag and it lets him hear whatever is going on around Montag.
Montag is going to use the device to help subvert the society he lives in. Faber will, for example, tell him what to say to people. He will also talk to Montag to help him understand and resist things that Beatty says to him (that are meant to force Montag to conform). So Faber will kind of talk Montag through his days, helping him to try to make people doubt the society they live in.
Faber's device is a tool for communication. Specifically, it enables communication between two people when these people are separated by distance.
In terms of its appearance, the device is about the same size as a ".22 bullet" and is green in color. Montag also notes that it looks a bit like a Seashell radio.
To use the device, Montag places it into his ear while Faber inserts a similar device into his own. The device then allows the two men to listen and speak to each other, even though they are not in the same room.
Montag intends to use the device against Beatty. Specifically, he wants to counter Beatty's pro-censorship rhetoric. For Faber, this device offers him the opportunity to snatch a glimpse into the world of the firemen and, more importantly, a chance to help and support his new friend and ally, Montag.