Placeholders are just what they sound like. They hold the place in an SQL query for a SCALAR variable from somewhere else in the script to be plugged in. EX:

#! /usr/bin/perl

use DBI;

print "Enter the city you live in: ";
chomp( $city = <STDIN> );
print "Enter the state you live in: ";
chomp( $state = <STDIN> );

$dbh = DBI->connect(your db info here);
$sth = $dbh->prepare( "SELECT name WHERE city = ? AND state = ?" );
$sth->execute( $city, $state );
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In this code the first placeholder(?) would be filled with $city and the second would be filled with $state. They are filled in the order that the variables appear as arguments to the 'execute()' function. -kel