Knowledge Items (KIs) are all the files you've added to Saganworks. They can be a variety of file types, including but not limited to: audio, video, images, documents, and web URLs.
For a list of supported file types, check out Supported File Types
There are many ways to add a KI to your Sagan. Visit this article if you've already upload your KI to Saganworks, and read these articles if you want to create a new KI directly from your desktop or from your phone.