Once we have the output tree, we need to fill it with the content of the
input tree. As you may have guessed, the <xsl:apply-templates>
element allows us to do precisely this. Think of it as a conduit from the
input tree to the output tree. It instructs the stylesheet to replicate the
content of the input tree within the output tree that you have created in
your stylesheet. In this case, it grabs the content of the matched
<paragraph>, Hello, World!, and puts it in the
appropriate place in the output tree (in the HTML <p> element).
The processor works much like the dog playing fetch in this slide. The
<xsl:templates> instructs the processor to fetch the content (in
this case, text).
We can imagine more complex examples, where the content of <paragraph>
is not only text, but additional elements that one may want to process in
some way. A more complex example like this will be covered in the next
slide.
