According to Harper, what is a "thread"? What does "lurking" mean? What does he help the FBI understand about EROS?

Harper explains to Dr. Lenz and the FBI agents involved in the "Lilith" act that a thread is a topic of conversation to which anyone can post a comment. He also explains that lurking is observing conversations without participating. He tells them that there can be anywhere from a few dozen to hundreds of rooms available for the killer to be in. Some are created by subscribers, and some are created by the system operators.