What it means is that they only pay for generation that's renewable and the energy company assures them by contract that they'll only be paying for renewables.

The rest of us, who might be on green tariffs usually get a commitment that on average almost all our energy comes from renewables. When supply is short then I'm the one who's paying for gas generation rather than the company who has committed to greenwashing. But that also drives investment in renewable generation, in the UK renewables have gone from 2% of power generation in 1991 to around 50% now.