The Editor
When editing page blocks, news stories, profile biographies etc you are able to do so in a WYSIWYG editor. This allows you to format text by adding headings, making text bold or italic, adding lists, links, emojis 
etc.
You can also embed tables and images in the text.
Note that the editor has its own (simple) styles. Your site styles get automatically applied to your text once you save it.
Headings
It's important to structure your text well for three reasons.
Readability
Breaking up long screeds of text with headings describing the content makes your pages more readable.
Accessibility
Correctly laying your page out makes it easier for people who use screen-reader software to navigate it, more quickly finding the content they're searching for.
Search Engine Optimisation
Search engines will use your headings to help determine what your page is about and improve your rankings.