Hi Dave and welcome to DC. One of the administrators could probably answer this better than me. But a thread is similiar to a "conversation" about a specific topic. Such as "cakes", "fishing". Hope I haven't confused you. Again, Welcome to DC, its a great place.
Welcome to the site. These members have given you a great explanation. To take it one step further, within each thread there are posts. Your post was the one that started this thread. Stinker's post was next and then mugsy27's post was after that. A thread is just a collection of these posts about a particular subject.
Ciao Dave!! I can't really think of anything to add to these fine answers above, just wanted to give you my welcome!! Have fun, don't be shy, if you have ANY questions, be it functions of the forum or cooking or food, ask away, we are here to help!