A faceplate is usually a Transformer's head unit, and may or may not be their mouths or vocabulators. With some bots the faceplate is removable or, can be automatically retracted to allow more efficient speech. In some if not all continuities, the faceplate is seen almost exclusively on Primes, inferring that the plate may be a sign of Prime rank. There are several exceptions to this rule, however.
Optimus Prime appears to have it as part of his face, but the last three have retractable face plates. In Revenge of the Fallen, Optimus' faceplate is destroyed when he is punched by Megatron. It is however restored when Prime is brought back by the Matrix and is combined with Jetfire's parts.