A TextTemplateRenderer is one which is configured by taking in a single text-based template file, and producing an output based on this text template. It is typically used to produce custom html, xml, or other text-based output by taking in a template file that contains static layout and content and which can render dynamic data within the report via scripting in either Groovy, Velocity, or another syntax.
For technical guidance, please consult the reporting module unit tests, particularly https://github.com/openmrs/openmrs-module-reporting/blob/master/api/src/test/java/org/openmrs/module/reporting/report/renderer/TextTemplateRendererTest.java