A "SceneCollection", as the name implies, holds all your scenes and sources (as well as global sources) that have been added to it. By default you start with just one collection. But if you have a very big amount of scenes setup for different events / tournaments / meetings or whatever you are doing, sorting them into different collections could be very useful.
You can then switch between the different collections using the menu shown above. Be aware, you cannot switch a Scene Collection while a preview, stream, recording or buffer is running.
To add, duplicate (copy), rename or remove a "SceneCollection" just use the different menu options. It is also possible to export or import collections. In this case be aware that sources, for example an image, will save the full path to the collection. On the PC, on which you plan to import the collection, you have to either update those paths or have the image saved at the same location.