Hello there Zoe,
I suppose it's easy to say a thread is a subject on its own - like this one here is called LURKERS UNITED and generally and at any time anyone who is a registered member of BOL (and registration to any one of the sites is very easy) can post a message. Generally what they say will be pertinent to the subject i.e. in this case lurking.
But, if someone wanted to know about growing microgreens for instance, they will not look under lurkers united but would go to the thread/forum subject opened up for that subject/topic.
The thread then follows, each thought or opinion or bit of advice etc. etc. and so on is then a thread of information... hope that makes sense to you, sometimes words get in the way! Go well today, cheers