What is the difference between Selected Element and Other Element?

These are advanced settings used when selecting a step while creating a Flow.

Selected Element: This is the UI element on which an action takes place. For example, the selected element could be a button or a menu that is clicked.
Other Element: The other element is any element that is present on the page on which the step is being created. It's used when the selected element does not have a unique identifier. So the step can be made to display based on the presence of the other element.