How to use prepared statements with Postgres

It means it will help you prevent SQL injection attacks by eliminating the need to manually quote the parameters.

Instead of placing a variable into the sql you use a named or question mark marker for which real values will be substituted when the statement is executed.

Definition of PDO from the PHP manual:
'The PHP Data Objects (PDO) extension defines a lightweight, consistent interface for accessing databases in PHP.'

See the php manual on PDO and PDO::prepare.

An example of a prepared statement with named markers:

<?php
$pdo = new PDO('pgsql:dbname=example;user=me;password=pass;host=localhost;port=5432');

$sql = "SELECT username, password
FROM users
WHERE username = :username
AND password = :pass";

$sth = $pdo->prepare($sql);
$sth->execute(array(':username' => $_POST['username'], ':pass' => $_POST['password']));
$result = $sth->fetchAll();

An example of a prepared statement with question mark markers:

<?php
$pdo = new PDO('pgsql:dbname=example;user=me;password=pass;host=localhost;port=5432');

$sql = "SELECT username, password
FROM users
WHERE username = ?
AND password = ?";

$sth = $pdo->prepare($sql);
$sth->execute(array($_POST['username'], $_POST['password']));
$result = $sth->fetchAll();

What do prepared statements mean in the statement?

From the documentation:

This feature allows commands that will be used repeatedly to be parsed and planned just once, rather than each time they are executed.

See pg_prepare

Example from the page linked above:

<?php
// Connect to a database named "mary"
$dbconn = pg_connect("dbname=mary");

// Prepare a query for execution
$result = pg_prepare($dbconn, "my_query", 'SELECT * FROM shops WHERE name = $1');

// Execute the prepared query.  Note that it is not necessary to escape
// the string "Joe's Widgets" in any way
$result = pg_execute($dbconn, "my_query", array("Joe's Widgets"));

// Execute the same prepared query, this time with a different parameter
$result = pg_execute($dbconn, "my_query", array("Clothes Clothes Clothes"));
?>

The MySQL documentation for Prepared Statements nicely answers the following questions:

• Why use prepared statements?
• When should you use prepared statements?