The Level Editor provides the core level creation functionality for Unreal Editor. This is where levels are created, viewed, and modified mainly by placing, transforming, and editing the properties of Actors. In Unreal Editor, the scenes in which you create your game experience are generally referred to as Levels. You can think of a level as a 3D environment into which you place a series of objects and geometry to define the world your players will experience. Any object that is placed in your world, be it a light, a mesh, or a character, is considered to be an Actor. Technically speaking, Actor is a programming class used within the Unreal Engine to define an object that has 3D position, rotation, and scale data. For sake of ease, however, it will be easiest to think of an Actor as any object that can be placed in your levels. At its most basic level, creating levels boils down to placing items in a map inside Unreal Editor. These items may be world geometry, decorations in the form of Brushes, Static Meshes, lights, player starts, weapons, or vehicles. Which items are added when is usually defined by the particular workflow used by the level design team. Since the interface for Unreal Editor is highly customizable, it is possible that what you see may change from one launch to the next. Below, you can see the default interface layout: The Level Editor has a tab along the top providing the name of the level currently being edited. Tabs from other editor windows may be docked alongside this tab for quick and easy navigation, similar to a web browser. The name of the tab itself will reflect the level currently being edited. This is a pattern consistent throughout the editor – editor tabs will be named after the current asset being edited. The Menu Bar in editor should be familiar to anyone who has used Windows applications previously. It provides access to general tools and commands that are used when working with levels in the editor. The Console (`) is a text field that allows special console commands to be entered that are recognized by the editor. The text field has an auto-complete feature that automatically lists all commands matching the text currently in the box. The Toolbar panel, like in most applications, is a group of commands providing quick access to commonly used tools and operations. This panel contains a set of viewports, each of which can be maximized to fill the entire panel and offer the ability to display the world from one of three orthographic views (Top, Side, Front) or a perspective view giving you complete control over what you see as well as how you see it. 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. The Modes panel contains a selection of various tool modes for the Editor. These change the primary behavior of the Level Editor for a specialized task, such as placing new assets into the world, creating geometry brushes and volumes, painting on meshes, generating foliage, and sculpting landscapes. You can close any panel by clicking the small ‘X’ in the upper-right corner of the tab. You can also hide any panel by Right-clicking on the tab, and then clicking Hide Tab on the context menu that appears. To once again display a panel that you have closed, click that panel’s name on the Window menu. The Modes represent different editing modes the Level Editor can be put into, enabling specialized editing interfaces and workflows for editing particular types of Actors or geometry. The World Outliner panel displays all of the Actors within the scene in a hierarchical tree view. Actors can be selected and modified directly from the World Outliner. You can also use the Info drop down menu to turn on an extra column that shows Levels, Layers, or ID Names. Layers provide the ability to quickly select as well as control visibility of groups of related Actors. You can use your layers to quickly un-clutter a scene leaving only the geometry and Actors that you are working with. For example, you might be working on a building that has multiple levels but is comprised of many modular parts. By assigning each floor to a layer, you can hide each of the floors you are not working on making the top view much more manageable. Source.