Apple designed the San Francisco family of typefaces to provide a beautiful, consistent typographic voice and reading experience across all platforms. In iOS 9 and later, San Francisco is the system font.
San Francisco works hand in hand with Dynamic Type to give you:
A range of font sizes that automatically provide superlative legibility and a great reading experience at all user settings, including accessibility settings
Automatic adjustments to letter spacing (tracking) and line height (leading) for every font size
The ability to specify different text styles for semantically distinct blocks of text, such as Body, Footnote, or Headline
Text that responds appropriately to changes the user makes to text-size settings (including accessibility text sizes)