You use Styles in LibreOffice to define the layout of pages in your documents. In Writer, the word processing module, you use styles to define your margins, fonts, font types, font effects, text flow including widow and orphan control and hyphenation. You can create or modify styles for single documents or choose to automatically update styles in all your LibreOffice documents created from the same template.
Styles come in handy when you are creating long documents with many sections or chapters like when writing a book, thesis or business report. Using styles consistently will allow LibreOffice to automatically create a table of contents.
You can even assign styles to individual paragraphs. Suppose you want to have certain paragraphs indented or vice versa, you want all paragraphs indented but other paragraphs like notes or explanations blocked to the margins. You would need two styles. One for the regular paragraphs with their margins or centering defined and another for your block text. Simply select your text and double-click the style you want to use for it. You can find more detailed information in the Instructions for Using Writer at
The User Guides are here:
Chapters 6 and 7 of the Writer Guide are especially helpful for styles and working with styles.