Every American citizen is given a portion of the budget they can appropriate as they please. This portion is equal for every American. The total budget appropriated to the citizenry might not compose the entire budget, it may only compose 25% of the budget, or less, but it gives Americans some direct control over the political process.
Only Americans who vote are given this power; Americans who do not cast their ballots do not get some amount appropriated to them.
Imagine if you could decide what you wanted your money spent on. Suppose you have $1000. If you like space travel but not the military, you can give more money to NASA and less to the Pentagon. If you don't like abortion, you can give less money to Planned parenthood.