Accordingly to the text of Theravada, the Buddha didn’t discuss about it much and it’s not known in Theravada tradition. It was developed after the Buddha death.
The realization of the emptiness of phenomena (suññatānupassanā) is an aspect of the cultivation of insight (vipassanā-bhāvanā) that leads to wisdom and inner peace.
In Pali, there are three doors of liberations, and they are: Shunyata, Animitta and Apranihita. Sunyata means emptiness. Emptiness always means empty of something. Here, I will discuss about sunyata only, and not the other tow. I think the meaning of the liberation and emptiness is similar. The essence of emptiness is liberation and liberation is also emptiness. For example: Nibbhana. It is the ultimate goal of Buddhism but there is nothing. This is in brief what I understand the emptiness in Theravada Buddhism.