Hi John, the URL is the internet address so to say ... the string of letters and/or numbers showing up in the browser line (top of the browser, the window in which you are viewing websites) telling where exactly in the internet you are.
For your last post it is "http://www.ulsterclans.com/phpBB/viewtopic.php?p=3626#p3626" - had to put it in quotes to show you how it looks originally, without quotes the software here will turn it into a nicer looking link when displaying it
If you click in that browser line once it will turn blue, [ctrl]+[c] pressed simultaneously will copy it and with [ctrl]+[v] you can copy it wherever you intend to, to let people know what internet address you are referring to.
That's the same all over the internet, the specialty of phpBB forums is that not only complete pages have their URL, but each and every single post is given its own unique URL, so you can refer your readers to exactly the one post you're speaking about, instead of having to tell them "this page, and post number so and so on it".
To retrieve the URL of the post you mean just click the wee sheet symbol at top of it and the URL will show up in the browser line, from where you can copy it and then paste it to your own contribution.
Hope this helps a bit