Database constraint violation?

$q = $db->query('INSERT INTO news (news_id, news_title, news_content, news_date, news_poster) VALUES (?,?,?,?,?)',
				 array($news_id, $news_title, $news_content, $news_date, $news_poster));

when i try executing that code in my script i get the error:

“DB Error: Contsraint Violation”

what does this mean? and what can i do to fix it?