How deep do you want the explanation to be, Meme? In this context, a "transaction" is a unit of communication between speaker and listener; it seems to be closer to the bi-directional "roundtrip" model of the way web browsers and web servers work than the atomic transactions involved in finance, or in other words a single chunk of language from sender to receiver. But basically what you quote is emphasizing that human language (as distinct from computer language and protocols) has the purpose of transmitting information. That seems self-evident to me, with the transmission of emotion as simply being a special case of the transmission of info. I don't know whether that was the sort of thing you were looking for. :)