The objective is to have the states vote their delegates in relation to the popular vote within that state rather than all of the delegates going to the overall winner of the state. It would have the states divide the delegates proportionately rather than en-bloc.
They recognize that changing the constitution to effect the popular vote is hard so they hope to have the individual states move towards this goal by splitting the delegates in a manner that more closely follows the popular vote count.
Any state that wants to can do so. It is entirely up to the state legislatures how to appoint their electors and even whether to have any sort of election to do it.
Here in NY, they would have had to give 35% of The electoral votes to trump. They won’t abide that here.