It would be all voters across the country electing the president, regardless of which state they live in. Right now, you have state electors directly electing the President, and certain states have extremely disproportionately low influence, and those states don't even secede.

It seems unlikely to me, but perhaps some states would try to secede if the system changed such that their voters receives the same influence as every other voter in the country. I would not support that decision.