Styles are preset styles for normal, formatted, and heading text. They allow you to easily keep continuity in your text formatting.
To apply a style:
Select the text you want to apply the style to.
Click on the Apply Styles button at the top of the window.
A small window will open with the list of available styles. Each style is displayed using that its own style settings. Click on the Apply Style that you want.
The window will close, and the style will be applied to your selected text.