Variables act as placeholders in your Topics that can be filled with changeable data. Say for example, you need to produce a document for your many different clients you can save time by producing one document, and then changing the Variables (company name, logo, and so on...) to make personalized versions of the document.
Variables are used in Topics, or can be assigned as metadata to Topics and Books. When variables are added to topics their names appear between angle brackets - <variable_name>, and the value of a variable is only displayed when the final output is published.