Answer: Literally it means "full voice." It can be taken to mean singing in the full "chest voice," i.e. not falsetto (a high part might in old music be sung in a light high tone by a man singing falsetto (think the Beach Boys) or the same pitches could be sung by a tenor singing in plena voce (think Pavarotti). The term doesn't just apply to voice but can be extended by analogy to other instruments: the full sound of which the instrument is capable.