Each page of an iStudio Publisher document is actually formed from a combination of two pages - a master page and a body page. Think of these as forming separate layers, with the body page layer always lying on top of the master page layer.
There's always a separate body page for each page of a document. You're likely to spend most of your time working on the body pages and place all of your non-repeating document content on them. For this reason, Body Page View is the default Page Mode setting when you open a document.
The current Page Mode setting is indicated by the color of the Page Mode buttons (on the page view title bar). As shown here, the blue filled Body Pages button is the current selection:
The canvas (page surround) color is also different in each Page Mode, to help keep your bearings.
Switch to Master Page View by choosing menu option View > Master Pages (or keyboard shortcut ⌃⌘M Control-Command-M), or by clicking the Master Pages button.
Switch back to Body Page View by choosing menu option View > Body Pages (or keyboard shortcut ⌃⌘B Control-Command-B), or by clicking the Body Pages button.
Master pages are provided for two main reasons; for convenience and to help achieve layout consistency. They are provided for holding any repeating content you want to include on multiple pages, such as headers, footers, page numbers and watermarks*; and for holding standard content in documents that may be copied and repurposed. You generally won't spend much time editing the master pages, and for some documents may prefer to leave them blank.
*iStudio Publisher does not include a formal way of entering a watermark. Watermarks refer to image or text objects that are usually added in the background, are often of partial opacity, and generally repeat from page to page.