We'll start with a simple example of an exchange containing two adjacency pairs. For ease, we'll show them in different colours, with pair 1 in red and pair 2 in black , like this:
In that little exchange we have two pairs as you can see and they form an orderly queue so that the conversation, such as it is, remains coherent. If Peter's first and second responses are swapped around, the conversation would become immediately bizarre and impenetrable. Adjacency pair 1: greeting – greeting response Adjacency pair 2: ask for information -–supply information The note that Peter's second response is preferred signals that there is another alternative which is dispreferred and that might be: I don't know which changes the second pairing to: ask for information–profess ignorance and that is not usually what the questioner wants.
It is not claimed here (or by anyone who knows a little about language) that all conversations develop via the use of predictable adjacency pairings but it is certainly true that a good deal of conversation between people would not be possible unless they form substantial parts of the exchanges.