What nonsense is that? Root is an element sui generis that represents the top of the element tree, that is, all elements descend from it. Most, if not all, functions are recursive down the element tree (except where propagation is disabled), so setVehicleColor(root, ...) would, while returning false (because the function makes no sense on its principal element, root), set the color of every vehicle that descends from it, or in other words, every vehicle in the world. Then there's resource root, from which all elements spawned by one resource descend. These elements can be recalled with getRootElement and getResourceRootElement (and the pre-defined variables root and resourceRoot). You can read more about this on the MTA wiki page for the Element tree.
Source is the element on which an event is triggered, an element that is the source of that event. Typically this will be the subject of the event name, for example, in the event onPlayerEnterVehicle, the player is the grammatical subject, and is therefore the source of the event. In an event handler -- that is, the function that is called when the event triggers (registered with addEventHandler) -- MTA defines a local variable called source that references said element. For example, in onPlayerEnterVehicle, the source variable references the player element of the player that entered a vehicle. The vehicle's element will be in the event handler's parameters. To look up what the source of an event is, and the parameters of each event, look up the wiki page for that event.