UBI is an unconditional cash transfer, which means that you get money from the government to spend however you want. That’s an unusual government spending program. In the US, besides Social Security, the government usually either spends money on a service (like health care or education) or gives conditional cash in the form of things like foodstamps.
But the government also spends a lot of money each year on cash transfers through “tax expenditures,” which is the money the government doesn’t collect in taxes because of exclusions in the tax code. Except for the Earned Income Tax Credit, those expenditures almost always help the rich more than the poor. By replacing them with UBI, we would create a more progressive system. That, not the elimination of all government programs, should be the starting place for debates about UBI.