Scenes contain the objects of your game. They can be used to create a main menu, individual levels, and anything else. Think of each unique Scene file as a unique level. In each Scene, you will place your environments, obstacles, and decorations, essentially designing and building your game in pieces.
When you create a new Unity project, your scene view will show a new Scene. This is an untitled and unsaved scene. The scene will be empty except for defult objects - either an orthographic camera, or a perspective camera and a directional light, depending on whether you started the project in 2D or 3D mode.
To save the scene you’re currently working on, choose File > Save Scene from the menu, or pres Ctrl/Cmd + S.
Scenes are saved as assets, into your project’s Assets folder. Therefore they appear in the Project Window, just like any other asset.
To open a scene, in order to begin or continue working within that scene, double-click the scene asset in the Project Window.
If your current scene contains unsaved changes, you will be prompted to save or discard the changes.
It is possible to have multiple scenes open for editing at one time. For more information about this, see Multi-Scene Editing.