The viewport shows your scene as currently rendered. You can freely move around the scene by manipulating the Editor's current camera.
Initially the Editor is set to use the Perspective camera. This camera is as if a movie camera was floating in your scene. You can use the camera dropdown menu to view the scene using various other cameras. The orthographic cameras: Top, Bottom, Front, Back, Left, Right, let you view a version of the scene with no perpspective. Useful for fine-tuning positions.
You can also use the camera menu to select any of the camera Entities in your scene. This way you can position your in-game camera exactly as required.
The 3-Colored Axis you can see in the screenshot above is called a Gizmo. This is used to manipulate the transform matrix of the selected Entity. There are three types of Gizmo: Translate (with arrows on the ends of the axes); Rotate (which is made up of three colored rings) and Scale (with cubes on the ends of the axes).
Entity Context Menu
Right clicking on an Entity in the 3D viewport will bring up the Entity context menu.