A text variable is an item you insert in your document that varies according to the context. For example, the Last Page Number variable displays the page number of the last page of the document. If you add or remove pages, the variable is updated accordingly.
InDesign includes several preset text variables that you can insert in your document. You can edit the format of these variables, or you can create your own. Some variables, such as Running Header and Chapter Number, are especially useful for adding to master pages to ensure consistent formatting and numbering. Other variables, such as Creation Date and File Name, are useful for adding to the slug area for printing.