It is way to store information (in variables). In order they are available in multiple pages for the same web application.
Example:
<?php// Start the sessionsession_start();?><!DOCTYPE html><html><body><?php// Set session variables$_SESSION["favcolor"] = "green";$_SESSION["favanimal"] = "cat";echo "Session variables are set.";?></body></html>
<?php
// Start the session
session_start();
?>
<!DOCTYPE html>
<html>
<body>
// Set session variables
$_SESSION["favcolor"] = "green";
$_SESSION["favanimal"] = "cat";
echo "Session variables are set.";
</body>
</html>