Recent posts - How do I show the most recent posts on the main page of my website From Online Manual
SSI.php is included in SMF. To use it, you have to use the following code on the main page of your website:
<?php require_once('/absolute/path/to/SSI.php'); ssi_recentTopics(); ?>
Please see ssi_examples.php, located on your forum's root directory, for more examples.
For more information on SSI.php, take a look here.
If you are not using a separate webpage for your homepage and people land on your forum index, you can show recent posts on the default forum index as well by going to Admin > current theme > Number of recent posts to display on board index