I believe they’re referring to an undefined threshold of buying power. E.g. if I earn $3000 but my take home is $200 after taxes, rent, food, utilities, and student loan repayments, abusive price hikes on basic needs could reduce my take home below the point of sustainability, even factoring in an extra $1000 on top of that. Basically, if rent, food, and utilities go up by 50% but I’m only earning 33% more.
Might be an extreme example, but I think it’s certainly a consideration that needs to be made when putting together the legislation. There needs to be some form of price control, otherwise those UBI checks could basically just become a free gift from the government to exploitative corporations and landlords.