Styles feature in word allows an author to have predefined formatting for differing parts of their document. Your title, subtitle, headings, sub headings and main body text should all be uniform and you may want them all to look slightly or majorly different with underlines, or bold font, or different font face, or size and colour.
Styles allows you to have these pre-set for easy selection. Word has a good number of default styles to get you started.
You can see a full list by selecting the down arrow on the styles ribbon, or select the small arrow to the right bottom of the styles ribbon.
Highlighting each style gives you an overview of the formatting.
You can update an existing style to your preferred choice or create an entirely new one.
If you want to change the heading 2 style displayed above from 13pt to 26pt for example. Select the style so it is highlighted in the ribbon, then change the size manually then right click the style and select update.
Or you can simply right click and select modify from the ribbon.
Alternatively you may just want to create new styles from scratch.
Type your new text with your own formatting then right click the text and select stylesàcreate style from the popup formatter.
Give the style a name and it will be saved for the future.
You can even use find and replace features to change styles in a document in one go.