Styles are such a timesaver and a lot of people don’t even know they are there. When you are writing pages and posts there is a dropdown that usually says paragraph.
Select some text and click on that dropdown, there is a list of predefines formats called styles. In the page or post edit screen they usually do not look like they will on the finished site but you can use the preview button to see how it will look.
Styles are set by the theme and when you apply a new theme as long as it has the same style the new theme will apply its format to the text. For example the style of Heading 1 can look different in different themes, so you do not have to go into each page to manually change the font or colour. This saves you a lot of time when you want to change the look of your site.