Here's how I explain declarations in Web pages to laypeople.
It's the equivalent of me telling you that "This post is written in 21st century American English." Yeah, that's kind of obvious to you, but computers are much more literal than we are.
This is why the first code at the top of my Media and New Media Ideas page is just a bunch of gibberish:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
That's because it wasn't written for people. Metadata is written instructions for machines. Browsers, search bots, feed readers, you name it.
And as we move up the food chain, it's possible to make those instructions more and more active.
So in theory, if I declare that the values contained in the RDF tags are contained at the directory https://dansExampleSemanticDirectory.com, then a bot crawling those tags will read from that directory when assigning meaning to the text.
At some point, there may be many directories of meaning out there on the web. I may have to declare all of them. I may have a root directory of meaning, and multiple subdirectories. I might just declare one directory and make all its tags dependent, or I might do that for some tags and list full URIs/URLs for everything that's contained in other cooperating directories.
It's difficult to say now, because we haven't developed these conventions yet.