A US citizen or US LPR whose income exceeds 125% of HHS poverty guidelines as specified here:
https://www.uscis.gov/i-864p
For example, if it's somebody with household size of 2 living in one of the 48 contiguous states, the 125% is $25550. So if the sponsor makes over $40000 it should be easy, if between $30000 and $40000 it's a little harder, and if it's below $30000 it may be a challenge. If it's below $25550 then they're not qualified.
The more dependants the person has, the higher income they need to be a sponsor.