A template is a document that contains specific formatting styles, graphics, tables, objects, and other information. A template is used as the basis for creating other documents. For example, you can define paragraph and character styles in a document, save the document as a template, and then use the template to create a new document with the same styles.
You can set a default template, so every new LibreOffice document would use it, unless you specified otherwise (for example, when you create a new document from a different template).
LibreOffice has a number of predefined templates that you can use to create different types of text documents, such as business letters.