Unique donors across (I believe?) all 50 states. It's to allow presumably grassroots candidates to achieve access. You can also qualify the normal way of scoring above some small percentage in national polls.
The DNC doesn't require donors from all 50 states, but it does require a healthy diversity of states. I think it is 200 unique donors from 20 different states.
Ha! For the record I’m all for it. I find the entire election procrsss highly entertaining.
The funniest thing I heard on the subject was the suggestion to nominate Trump as a Democrat so he qualifies for their primary debates and can take on all of them at once.