What is the SnackBar widget?

A SnackBar is a brief message that appears at the bottom of the screen to provide lightweight feedback about an operation (e.g., "Item deleted"). Show it using ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text("Message"))). You can add an action (like "Undo") to the SnackBar. It automatically disappears after a short duration, or you can set a custom duration. SnackBars are queued — only one is shown at a time.