A “public charge” as defined by USCIS is a person who is likely to become “primarily dependent on the government for subsistence, as demonstrated by either the receipt of public cash assistance for income maintenance or institutionalization for long-term care at government expense.” – basically, a public charge is a person who cannot support themselves and depend on benefits that provide cash. People who are likely to become a public charge is not admissible to the US and is not eligible to receive a permanent resident card. However, it’s up to the government to try to predict if anybody will need the proscribed services – it’s not based on if they are already getting it but that they think the intending immigrant is likely to need those benefits.
Benefits that provide cash include Supplemental Security Income (SSI) and Temporary Assistance for Needy Families (TANF). Benefits that do not provide cash – such as the Supplemental Nutrition Assistance Program (SNAP) aka Food Stamps or subsidized housing do not factor into considering a person a public charge.