“Religiously Unaffiliated” is a bit of a catch-all term that can be used to refer to those who are still religious or believers but who do not adhere to a particular orthodoxy or religious institution or denomination.
However, in this case, given the language used, I think we can safely say that it refers at least in large part to those nonbelievers who scatter the country and, more often than not, vote Democrat.
The DNC recently (August) produced a resolution that is actually pretty landmark, in my books. Here it is in full:
DEMOCRATIC NATIONAL COMMITTEE:
Resolution Regarding the Religiously Unaffiliated Demographic
WHEREAS, the religiously unaffiliated demographic has tripled in the last two decades, now representing 25% of the overall American population and 35% of those under the age of 30; and
WHEREAS, religiously unaffiliated Americans overwhelmingly share the Democratic Party’s values, with 70% voting for Democrats in 2018, 80% supporting same-sex marriage, and 61% saying immigrants make American society stronger; and
WHEREAS, the religiously unaffiliated demographic represents the largest religious group within the Democratic Party, growing from 19% in 2007 to one in three today; and
WHEREAS, the nonreligious have often been subjected to unfair bias and exclusion in American society, particularly in the areas of politics and policymaking where assumptions of religiosity have long predominated; and
WHEREAS, those most loudly claiming that morals, values, and patriotism must be defined by their particular religious views have used those religious views, with misplaced claims of “religious liberty,” to justify public policy that has threatened the civil rights and liberties of many Americans, including but not limited to the LGBT community, women, and ethnic and religious/nonreligious minorities; and
WHEREAS, the Democratic Party is an inclusive organization that recognizes that morals, values, and patriotism are not unique to any particular religion, and are not necessarily reliant on having a religious worldview at all; and
WHEREAS, nonreligious Americans made up 17% of the electorate in 2018 and have the potential to deliver millions more votes for Democrats in 2020 with targeted outreach to further increase turnout of nonreligious voters; and
WHEREAS, a record number of openly nonreligious candidates are running for public office;
NOW, THEREFORE, BE IT RESOLVED, that the DEMOCRATIC NATIONAL COMMITTEE recognizes:
1. The value, ethical soundness, and importance of the religiously unaffiliated demographic, a group of Americans who contribute in innumerable ways to the arts, sciences, medicine, business, law, the military, their communities, the success of the Party and prosperity of the Nation; and
2. That religiously unaffiliated Americans are a group that, as much as any other, advocates for rational public policy based on sound science and universal humanistic values and should be represented, included, and heard by the Party.
Expect some targeted outreach.