The Voice of All Things refers to the words conveyed by inanimate objects and animals that don't speak the human language. Although these words are usually either inaudible or unintelligible to humans, there exists an innate ability that allows a person to perceive these words and to understand and gain information through them. For example, it is possible to use this ability to gain information from the Poneglyphs. Furthermore, some people are able to converse with the voice and, in case it belongs to an animal, might even be able to command it.
Those who are able to perceive the Voice of All Things are usually unaware of their ability and don't know about the Voice of All Things itself. When Roger first crossed the Red Line, he was able to perceive the Sea Kings' voices, but he didn't understand who was talking to him on the sea floor. However, by the time he reached the end of the Grand Line and began his search for Laugh Tale, he was able to perceive the voices of Zunesha and Big Mom's Road Poneglyph. Like Roger, Luffy is able to perceive and understand animals like the Sea Kings and Zunesha. Kozuki Oden and his son, Kozuki Momonosuke are also able to perceive Zunesha. Unlike Roger, Luffy and Oden however, Momonosuke was able to actually converse with the elephant.