A thread is a topic of discussion on the forum. You've just started your own thread right here.
Tag means you're it.
Ok, so in terms of the forums, it means placing html tags around part of your message. For example, if you want a line to be in Bold text, you can highlight it by left clicking your mouse and dragging the cursor over all that you want in Bold. Then click on the B just above this box where you type your message. That allows you to put B tags around the line.
Here is the example You can change font size, change color, use underlining, etc.
If you hit the Go Advanced button down below, you will have many more options on what types of tags to use.
Code tags are especially useful for long log files from burning or encoding programs that you want to post here. They don't take up as much room that way.