aria-describedby references one or more element IDs whose combined text becomes the accessible description of the element carrying the attribute. It is the standard way to programmatically connect a form field to nearby helper text, an error message, or format instructions that a screen reader should announce along with the field.
A frequent implementation bug is referencing an ID that does not exist anywhere on the page, or forgetting to update the reference when an error message is dynamically inserted, both of which silently break the connection.