In NVivo, memos are a type of 'source'. You create or import them in the same way as documents and externals.
You can create memos to capture your thoughts about data, concepts, research procedures and so on. When a memo is related to (or inspired by) a particular source or node, you can create a 'memo link'.
For example—you might link an interview transcript to a memo that describes your thoughts about the interview process.
You can link each project item (document, external or node) to one memo but that memo cannot be linked to another item. You cannot link a memo to another memo.
View and manage memo links in the Memo Links folder—in Navigation View, click the Links button.