It's to let fans put their stories in Star Trek Online with the same "look, feel, and capabilities" of the Cryptic team. You can't import your own assets though.
"The Foundry is Cryptic Studio's tool set for allowing players to create their own content for Cryptic's games. This is also called a Mission Authoring Tool. Star Trek Online will be the first Cryptic game to use this tool set," reads The Foundry FAQ.
"Players can customize pre-made planet surfaces, and star systems with their own encounters, objects, and story. They can also create their own star systems from scratch. More map features are planned for future updates."
It's hopefully a great way for the STO community to keep the experience fresh and avid fans (read: nerds) will be able to scrutinise new content for their fellows to explore.
"The goal of this tool set is to allow players to put their stories in the Star Trek universe using the same look, feel, and capabilities that Cryptic's own design team utilizes. So while there are a lot of features for creating content, players cannot import their own images, 3D models, or audio into the game," continued Cryptic; it's not a 'mod tool'.
These 'Community Authored Episodes' can be rated by players of Star Trek Online, but first everything put together will get a passing glance from the studio development team in case anyone tries to slip something through that shouldn't be there.
To be a part of The Foundry beta testing you need only an active subscription.