the templates_c sub-directory to your XOOPS site's root is a repository for "compiled" templates.
Templates are written using Smarty syntax, which resembles normal HTML to a great extent - however, for those templates to be usable by the system, they must be compiled to PHP code.

When a template is invoked for the first time, it is compiled to PHP code and placed in the templates_c directory for faster access on next invocation.

See also FAQ about Clearing cache