Ever heard a guitar talk? That's probably due to a vocoder.
What happens, using the talking guitar situation as an example, is that the amplitude characteristics of the voice (the modulator wave) are taken first from a vocal track. These characteristics are then applied to the guitar (the carrier wave) so that when
the voice is loud, the guitar is loud, and conversely, when the voice is soft, the guitar is also soft.
But what makes the vocoder sound like a vocoder is the fact that before it applies these amplitude characterists to the guitar, it splits up the sound of the voice and processes it in such a way that the filtering characteristics of the voice are also incorporated into the guitar sound. This concept is very difficult to explain, and, thus, if you would like to learn more, you probably need to search for an article on the internet that explains the vocoder in great detail.
But if you want just a general definition of one, a vocoder, essentially, takes the amplitude and filtering characteristics of one wave and applies it to another.