I maintain 2 legacy Symfony sites that make use of tpl files, in that site for creating ebooks from PDFs.
A tpl is a 'template file' that can hold information about the another file type that is to be created.
For the Symfony sites the .tpl files are used to create jpgs for displaying in the ebooks.
So, they aren't something restricted to one particular platform or framework and they can have many uses. How they work in your project is impossible to know for an outsider.