Accessibility traits on iOS is the system by which assistive technologies know how to present your interface to your users. The exact experience will vary between assistive technologies, in some cases they may change the intonation of what VoiceOver reads, or add additional options for navigation, sometimes they will disable that assistive technology from accessing the element, or change how the assistive tech functions. They are the ‘Role’ part of the fundamental rule of making something accessible to screen readers – WCAG’s 4.1.2: Name, Role, Value.