I haven't actually used them myself and don't know the best ways to use them. As I understand it they're a device to make searching easier.

When you start a new thread, under the text box there's a smaller box for entering tags if you want to use them:

Tags
Separate tags using a comma.
You may add 25 tag(s) to this thread.

When I hover my mouse pointer over one of those blue tag markers, a list of the tags for that thread appears.

In Advanced Search, there's a box to enter tags that you want to search for.

If most threads were tagged (and the tags were applied consistently from thread to thread) it could sometimes make searching easier. Hardly anyone uses tags, though, so they're only good for finding the few posts that actually have them.

I don't know offhand what the "tag cloud" at the bottom of the Advanced Search page means or does.

If you figure out any more about tags or find a thread about them somewhere, I hope you'll post the information here.