Define and explain template hierarchy system
WordPress uses a template hierarchy system to decide which template is used for a web page. WordPress will go down the hierarchy of templates until it finds the template which the admin selected and/or until it finds the template that matches the slug or ID from the URL.
Define and explain template hierarchy system order
As WordPress finds the appropriate template to use, it searches for the best match and will skip over other template files in the hierarchy in order to find the match. For example, when an admin creates a page, they select if it will be a blog post or a static page. That is one part of the hierarchy. If the admin chooses to make a static page, then a page template will need to be chosen. Depending on that choice, WordPress moves further down the hierarchy to select the default template or a custom one. If the default template is selected, you can still make a specialized page template that WordPress will automatically match with as long as you include the slug or ID in the file name. Once WordPress finds the best match for a file and template, it will then send the information to the page.php, singular.php, and index.php files.
Define and explain template hierarchy system naming conventions
File naming conventions are what possibly helps WordPress the most when finding the best template match for a file. It's important to keep file names simple. For example, if you were to create a specialized template for a page, it's important to still have the file name begin with the word "page" instead of something else such as "blog." If you're using a specialized template, it's also important to include the slug or page ID in the file name. Otherwise WordPress won't be able to match the template file with the page you want to use it on.
Summary
The WordPress template hierarchy system is simple, but yet very complex at the same time. The idea of skipping over templates until WordPress finds the correct template is easy enough to understand, but there are many different customization options users can use for templates which makes it complex.