I will repeat my understanding, and then I will ask Dr. Patrick to say it in better terms than I can.
There are two things. There are the publicly broadcast Wi-Fi signals that they knew they were picking up, and they intended to pick up. What they did not know was that in that collection, they were also picking up meaningful payload data, which means not only the signals themselves but the communications, contents of messages, that were being transmitted through those signals. That's the best way I can explain it.
Dr. Patrick.