Building a HTML Template and linking it other HTML pages Options

[tea-ch]

I'm studying for a HTML test and one part of the test requires us to "create an index.html file, link your CSS and link your template file"
I have created a CSS file and HTML "template.html" file.

I fully understand adding a link to my external CSS file, but I have never learning about linking a template to an HTML file.
This test is a bit on the older side of the technology, for example it requires us to build a table as a page layout (no so great )

So this answer maybe outdated practices, but I need to understand it (if it is even a thing)

Can anyone help???