Dynamic pages are pages which are automatically generated from a database. For example, an estate agent might want to list hundreds of properties on the website at any given time. The manpower cost of creating an individual page for each and every one would be huge, so the solution is to put that information into a spreadsheet and have the website automatically (dynamically) create pages based on that.
Before now, this meant maintaining a spreadsheet on Google Sheets or the excellent Airtable. Both options are still available but, as of today, you can create that database of information from inside SITEHUB itself. This means there's no additional cost, no need to train employees on third party software and no need to rely on software which could change its feature set or increase its cost at any time.