A negative income that is better than that. It says, if you're working, but only making $12k, the state will give you money so you now have $20k. (Not real numbers.)
The idea is that it incentivizes participation in the work force, with hopes that the extra money helps you get stable and move up the payscale where you may stop needing the external support.
No, negative income tax usually requires that you make some money and file taxes. UBI doesn't.
One has the intention of encouraging workforce participation. The other tries to help everyone.