Hidden
Checking the box 'Hidden' means that the scene will not appear visually in the Live view, but otherwise will operate exactly as if the scene was there.
You are not able to create a Dashboard for that scene (as it cannot be seen) and the operator cannot skip the scene. You can sill create Dashboard widgets in the Show settings that refer to inputs or outputs that are used by the hidden scene.
The hidden scene is useful if you want to create some behaviours that are tied to a particular moment in the show, but they need no feedback or controls on the operator end.
Optional
Checking the box 'Optional' means the scene doesn't need to be finished or cancelled in order to end the show. It's a scene that can remain incomplete. For more info on how scene's work see: Scene Lifecycle & Show Lifecycle
This is useful if you have scenes that are not integral to be competed in order to move the show on, or are extra scenes that the audience may or may not encounter during the show.