Culling mask are the layers you want the camera to see, in my case I only want it to see UI.
The Viewport is where this camera will show up on the camera so either in front of or behind another camera, think of it like layers. You can customize the viewport if you only want to show a small section of another camera or if you want to scale what the camera see's.
Depth is the order the camera shows up with, higher numbers appear above/in front of lower numbers. So a camera with a depth of 1 will overlay a camera of 0 or -1.