There are a few different things active all at once - not just objects! You can have multiple scenes, so the one that you're in is considered active. Each scene can have many layers, but you can only view one at a time, so whichever one you're looking at is considered active. You'll find the scene and view layer drop-downs on the right in the main header, but their properties are found in the properties editor. You can have different tools selected in the toolbar, so whichever one you're using is considered the active tool.
Hope that helps!
