I think it is hand picked by the WordPress.com team. You may observe what type of content gets featured there and try to write similar butoriginal, high quality and interesting content with nice images. Good luck!
The person behind the curtain is me, Joy Victory, the Editorial Czar for WordPress.com. Each weekday I go and look for the best posts on WordPress.com and add them to Freshly Pressed.
I’ll soon be posting more documentation on what it takes to get added to Freshly Pressed, and why it matters to your site — stay tuned. We also just added a Freshly Pressed feed that you can subscribe to: http://freshlypressed.wordpress.com/feed/