The Details panel contains information, utilities, and functions specific to the current selection in the viewport. It contains transform edit boxes for moving, rotating, and scaling Actors, displays all of the editable properties for the selected Actors, and provides quick access to additional editing functionality depending on the type of Actor(s) selected in the viewport. For instance, selected Actors can be exported to FBX and converted to another compatible type. The Selection Details also allows you to view the materials used by the selected Actors, if any, and quickly open them for editing.
See the Details page for a more complete overview and guide to using the Details panel in the Level Editor.