The Base Styles > Body section contains settings which control the styling of the body element. This usually applies to the base text.
The Text color is applied to the body element text. This applies to all text elements on your site, however it is usually overridden by styles with a higher priority. See the screenshot below.
The Background color setting changes the default background color of the site:
The Body Typography setting controls the base font settings for your site. Changing these settings will apply to all text on your site, but will be overridden by more specific settings.
The Font Size setting controls the base font size of the body element, which applies to the default paragraph text of your site. There are 2 options, Mobile and Desktop. Mobile controls the font size on mobile devices and Desktop controls the font size on Desktop devices.
Controls the default spacing above and below paragraphs. It is usually recommended to use only one of the margin settings, generally the margin bottom.