Margins and columns are nonprinting guides that can be set for pages and spreads and can be used to frame where columns of text or content may go.
To change margin and column settings for a page, navigate to a page in the Document window or select a spread or page in the Pages panel. Choose Layout > Margins and Columns, specify options, and click OK.
When you change the column and margin settings on a master page, you change the setting for all pages that the master is applied to.
Changing the columns and margins of pages selected in the Pages panel affects only the selected pages.