Explain semantic trees and accessibility in Flutter.

Flutter builds a parallel Semantics Tree alongside the Render tree to communicate with OS accessibility tools (TalkBack/VoiceOver).

Because Flutter draws its own pixels, the OS doesn't inherently understand what a 'button' or 'text' is. The Semantics tree maps Flutter widgets to OS-level accessibility nodes. You use the Semantics widget to explicitly provide labels, hints, and roles (like button, slider) for screen readers.