This field lets you provide additional "error text" specific to a question, row, or column (of a grid), to supplement the global default error text. The "error text" is the additional text that can be shown to the respondent if there is something invalid regarding this question. The text you provide here is returned if you use the [%ErrText( )%] script instruction when the survey runs on a server.
For example, if the question concerns the person's marital status, you could specify the error text as "(Please specify your Marital Status)".
Then, within the Missing Answer error message (on the Error Messages tab of the Survey Settings dialog), you add the script instruction [%ErrText( )%] to insert the text you supplied within the error message to additionally return the words: "(Please specify your Marital Status)".