Communication is the transfer of meaning from a sender to a receiver through a message carried by a channel, completed by feedback. If the receiver's understanding differs from the sender's meaning, communication has partly failed, however clearly the sender spoke.
Worked example: a teacher explains a concept using heavy technical jargon to first-year students. The students hear every word but understand little. The barrier is semantic: the words themselves block meaning. Speaking louder would not help; simpler words and an example would.