Apparently he's a proponent of Universal Basic Income, an idea which seems to be largely discredited, from what I can tell (read: what I can Google

).
I did try thinking about it, but discovered I'm nowhere near smart enough to see how it could possibly work.
Basically, as far as I can tell, this is what would happen:
Introduce UBI set to NMW (or living wage).
1. People quit their jobs, if their jobs are low-paid and menial. This includes factory workers, people emptying bins, etc.
2. Wages for these jobs increase drastically because we need them to be done.
2a. Alternatively, all these workers are migrants workers who do not receive UBI.
2b. Alternatively, services are withdrawn (bins are removed).
3. Prices increase for products affected by wage increases. This includes food production.
4. Employers rush to automate as much as possible to reduce wages (a good thing, esp for menial jobs).
5. Those on UBI find that due to increased prices, they are struggling again.
6. Tax!!! Where do we get the billions to pay UBI in the first place? Where do we get it when people stop working?
Now I'm sure John McDonnell has thought about this much longer than I have

And perhaps the system would find some kind of balancing point.
But I'm not sure how you would begin to model this system when it so drastically changes the fundamentals of our society. Whoever successfully decouples income from labour probably deserves the Nobel prize