this all relates to your dashboard. the dashboard is where you can layout and set up buttons to trigger your devices. after you have the dashboard configured, you can either log into the dashboard through a web browser on your iPad, or you can download the hubitat app and access the dashboard that way.
depending on how locked down you want it to be, there are kiosk type apps that you can lock the device down to only open certain apps or webpages (you would lock it down to the webpage of your dashboard)
you can create multiple dashboards, so if you wanted to separate dashboard for each room, then link to those rooms in another main dashboard, you can do so. this can be helpful if you plan on having multiple tablets for different rooms, and only want that room to be accessed on a specific tablet, but then you can access all the dashboards on your phone