To put a very complex subject in simple terms, dhvani is two things. It is the emotional meaning of poetry in the Pracina and Navina schools of poetry in Sanskrit. Dhvani is poetry that adopts the avenue of suggestion through emotion; it is not devoid of the concrete, like description, but it elicits suggested emotion as its core meaning.
More often called the Theory of Dhvani, this very complex subject starts with the difference between the expression and the meaning. Dhvani is the expression that bridges the two schools and gives the essence of each. Pracina school stops with the concept of expression, which leaves the story or narrative of the poem as an incidental in light of the words that embody it, the dhvani that embodies it. The Navina school carries dhvani further by suggesting a yielding to the emotion that is suggested by the dhvani thus giving the content, the story, a place with the embodying words.