The dock is the small toolbar at the bottom of the canvas, under your resume preview. It controls how you view the preview. It doesn’t change the resume or any exported file.

Controls

With a one-page layout, the dock shows 1 page instead of the page buttons.
The page buttons step through the pages of your layout (Design > Layout). To see how many pages the PDF will have, read the label above each page in the preview, for example Page 1 of 2.
Your browser’s own zoom shortcuts still zoom the whole app, not only the preview.

Where the other canvas actions are

The dock only holds view controls. The rest sits in the header above the canvas: Press ⌘ \ or Ctrl \ to show or hide the Edit panel and give the canvas more room.