The terms page and spread are fairly interchangeable in Affinity Publisher. If you're working with single pages rather than facing pages, the terms are interchangeable. However, for facing pages, the term spread becomes more significant.
At document setup, you can create pages and spreads for your document. Pages can be presented as single or facing pages, and can use a portrait or landscape orientation.
The Facing pages option allows you to arrange your publication in the form of spreads. By using spreads, instead of working on single pages, you'll be working with left (verso) and right (recto) page pairings displayed at the same time, e.g., a gatefold centre spread of a magazine. Spreads can be arranged vertically or horizontally.