As a U.S. lawful permanent resident (LPR or green card holder), you might be legally able to receive some public benefits, such as SSI, TANF, Social Security, Medicare and more. Exactly which ones you're eligible for depends on the length of your U.S. residence and the type of benefit (as described in What Public Benefits Can a Green Card Holder Receive?). However, that doesn't make receiving public benefits risk-free.
If you have relied on public benefits, particularly cash assistance, and you then travel abroad, your green card status could be put into jeopardy upon attempting to reenter the United States. You run the risk of being declared a "public charge" by U.S. immigration officials and thus losing your immigration status.
The legal reason for this is that a public charge can be deemed "inadmissible" to the U.S., in other words ineligible for U.S. entry or a green card. This won't affect you while you live in the U.S. with a green card; only after travelling overseas for a certain length of time (described below) and attempting to return.
A short trip outside the U.S. won't likely put an LPR's immigration status at risk. But receiving certain public benefits can cause problems for LPRs who remain outside of the country for more than 180 days.
When such a person tries to return to the U.S., immigration officials at the border, airport, or other port of entry will treat him or her differently than LPRs who've been away for less time. In legal terms, the returning LPR is viewed as an applicant for "readmission," who must therefore be inspected for "inadmissibility."
These grounds of inadmissibility are the same ones the LPR had to face when first applying for the U.S. green card; and one of them was the "public charge" ground. A person is considered a "public charge" if he or she is likely to become dependent on public benefits. Most people applying for their green card must show that they're either entering the U.S. based on a job offer or have received an Affidavit of Support from a family member, so as to prove to U.S. immigration officials that they'll have a source of income or care and are unlikely to need government financial assistance in the future.
But now, when the LPR is returning to the U.S. after a long trip, the border or airport officials can look to the past, not just the promised future—and see that the LPR became a public charge after all. At that point, the border official can initiate proceedings to have the person's lawful permanent resident status taken away.
It's also important to note that there are situations in which an LPR will be considered an applicant for admission even after having been outside of the U.S. for 180 days or less. For example, a permanent resident who "engaged in illegal activity" while outside the U.S. would be considered an applicant for readmission even if that person had been out of the country for only a few days. In such cases, public charge and other grounds of inadmissibility could also apply, making returning to the U.S. impossible or very difficult.
There are a few exceptions to who is subject to the "public charge" ground of inadmissibility. It does not apply to permanent residents who received their green card as refugees or asylees, as Amerasian special immigrants, and in some cases under the Cuban Adjustment Act (CAA), the Nicaraguan Adjustment and Central American Relief Act (NACARA), or the Haitian Refugee Immigration Fairness Act (HRIFA).
Not all public benefits automatically trigger the "public charge" bar to admission. Immigration officials are supposed to ignore the receipt of most types of non-cash public benefits, such as unemployment, school lunches, job training, Head Start, Social Security Disability (SSDI), and emergency relief when making a public-charge decision.
The government's determination isn't automatic. It will consider receipt of public benefits as part of assessing the "totality of circumstances" in someone's case. As part of this "totality," they will also take into account factors like the applicant's age, health, education, skills, family status, and financial assets and resources.
(By the way, the Trump administration's definition of a "public charge" was significantly stricter than that previously taken by the U.S. government, and put many more immigrants at risk of inadmissibility. However, federal courts said this was stricter than the law allowed, and put a stop to them. The Biden Harris administration did not attempt to continue the Trump Administration's court appeal, so those regulations are dead for now.)
For more information about public benefits and immigration status, see Will Receiving Public Benefits Hurt Your Chances of U.S. Citizenship?