Yeah, I don't really understand it either. But in this case, it tells the browser to add target="_blank" to any link that includes "http" in its URL. So, for example, you could use this on a site where you wanted any link that includes "http" to open in a new window (such as an external link) but not to do so for internal site links (which might, for example, be root relative links, and thus not contain "http").
If you just wanted all links to open in a new window, you could just use
All the same, as has already been discussed, this kind of behavior is not recommended now anyway, as it is confusing to users and kind of ruins the functionality of the web (the back button etc.).
If you are going to use tartet="_blank", it's a good idea to use an outdated transitional doctype, to help remind you that the site is using outdated code. Except that ...
Indeed, that's the case—which is consistent with the HTML5 motto of "paving the cowpats" [typo intended :)].