To understand this we first have to realise what scenes are. Scenes refer to the objects which contain a particular scenario of the game.
Consider you have a game that has a world consisting of 2 parts: a city and a forest. Here we can say that this world can be defined in 2 scenes (though for larger games this may not be the case), one for the city and one for the forest.
Quoting from Unity Manuals:
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.
Scince the few latest updates, Unity has started using a new library to access Scenes and traversing between them. This library is called the SceneManagement.
To use this library we have to first include it. We do this by writing
at the start of the script.
In this library the class that you will encounter and use the most will be the SceneManager.
To be able to access a scene via the SceneManager class we first we need to add the scene to the Build Settings
To do this save the scene by CTRL+S. It will be saved in the Assets folder.
Now go to File in the menu bar and select Build Settings.
In the window that opens up, drag and drop the scene icon from the Project tab into the space under Scenes In Build.
We can add multiple scenes in a similar way and reorder them. The order in which the scenes are, decides their scene number and the scene which will load first when the game begins.
The Scene Manager class has the following:
Scene management at run-time.
|sceneCount||The total number of currently loaded Scenes|
|sceneCountInBuildSettings||Number of Scenes in Build Settings.|
|CreateScene||Create an empty new Scene at runtime with the given name.|
|GetActiveScene||Gets the currently active Scene.|
|GetSceneAt||Get the Scene at index in the SceneManager's list of loaded Scenes.|
|GetSceneByBuildIndex||Get a Scene struct from a build index.|
|GetSceneByName||Searches through the Scenes loaded for a Scene with the given name.|
|GetSceneByPath||Searches all Scenes loaded for a Scene that has the given asset path.|
|LoadScene||Loads the Scene by its name or index in Build Settings|
|LoadSceneAsync||Loads the Scene asynchronously in the background.|
|MergeScenes||This will merge the source Scene into the destinationScene.|
|MoveGameObjectToScene||Move a GameObject from its current Scene to a new Scene.|
|SetActiveScene||Set the Scene to be active.|
|UnloadSceneAsync||Destroys all GameObjects associated with the given Scene and removes the Scene from the SceneManager.|
|activeSceneChanged||Subscribe to this event to get notified when the active Scene has changed.|
|sceneLoaded||Add a delegate to this to get notifications when a Scene has loaded.|
|sceneUnloaded||Add a delegate to this to get notifications when a Scene has unloaded.|
Courtesy: Unity Scene Manager