Currently, you can save text or upload files.
You can save "notes" which are rich text (with added formatting) - think of them as "knowledge articles" where you summarise what you know on a topic. The topic could be anything from your silver coin collection to your family's investment portfolio to a full how-to guide on configuring your website on a new server. These can be "nested" - i.e. one note can be a sub-note of another, so you can have an overview of a topic and then a "drill-down" into more specific sub-topics.
You can also upload files - images or documents. These go into "collections" and can also be referenced from notes. You can reference the same document from multiple notes, no need to re-upload it into each.