It means that MySQL was unable to process your query for some reason, so mysql_query() returned Boolean false instead of a query result resource. Instead of jamming everything into one line, try some defensive coding, instead.
$sql = "SELECT * FROM `Reports` WHERE `Username`='".mysql_real_escape_string($reporter)."'";
$result = mysql_query($sql);
if($result == false) {
throw new Exception("Query failed:\n".mysql_error()."\n$sql"); // or however else you like to handle errors
}
$reporterarray = mysql_fetch_array($result);