Thread
A thread is usually a flow of conversation on one topic. Messages are organised into threads to try to cut down the confusion of everyone adding comments at once. Let’s say Hermione posts a message and both Hagrid and Ron reply to her, then Harry replies to Ron, but at the same time a few other people are talking about the spells they learned last lesson, if you tried to read the messages in the order they were posted then it would be chaos! So usually messages are organised into threads, letting you follow the flow of a single conversation which may span weeks, months or even years.