it means the variable you are passing into the function did not result from a valid query, or the query failed and didnt create a resource.
$x = 7;
$row = mysql_fetch_array($x); //not valid
$q = mysql_query("ZZZZZZ"); //invalid query
$row = mysql_fetch_array($q); //query failed, invalid resource.
edit:
this explains resources: http://php.net/manual/en/language.types.resource.php
