PHP Warning: mysql_result() expects parameter 1 to be resource, boolean given

PHP Warning: mysql_result() expects parameter 1 to be resource, boolean given

I been trying to learn how to code and been looking at examples. In this
line i am getting the topic error can someone explain what this line does
and why it creates that error
$pages = ceil(mysql_result($pages_query, 0) / $per_page); //dividing
total rows with total data to