Welcome Lynn! You'll get the hang of things eventually. I believe Leigh was referring to the "built-in" help menu, found at the far right of the list of drop down menus found on the menu bar at the top of your screen. UI is short for User Interface—essentially the design of the software itself—all the buttons, tools, etc. that we as users interact with to make the software work.
I can understand the frustration. There is a lot of new information to gather in, even for those of us that are experienced in Photoshop and image editing. The concept of "personas" is unique to Affinity software, and took me a bit to figure out. A lot of the terminology that you are questioning is stuff that has been around since the days of printing photos with negatives and film enlargers and has carried over to digital photography and image editing.
I'm assuming you've already seen this video. If not, it may shed a tiny bit of light on things. Always feel free to reach out to us here on the forums. Plenty of folks ready to help out new users (which I am myself).