The affidavit of support, USCIS Form I-864, is a promise by a U.S. citizen or resident to provide financial support, if needed, for an immigrant to the United States.
If you are the relative who has petitioned for the person to immigrate, then you must also sign an affidavit of support for the person, in order for the person to be able to complete the immigration process.
The person signing the affidavit of support is required to provide documents to prove his legal status, a copy of his most recent federal income tax return & W-2 form, and information about his adjusted gross income for the past three years.
If the petitioning relative’s income is not sufficient, according to USCIS standard guidelines, to support himself and the immigrant, then an additional person, a co-sponsor, will also be needed. That person would have to also sign a separate affidavit of support, and provide all of the same type of documents and information.
Each person signing is entering into an enforceable contract, which may be used to require him to contribute funds for the support of the immigrant. It is unusual that such enforcement is done, but it is a responsibility that has been enforced by courts in the United States. The signer can be liable to reimburse agencies that provide benefits to the immigrant, among other possible liabilities. Such agencies might be given access to the information you provide on the form.
The liability will continue until the immigrant dies, becomes a US citizen himself, or acquires 40 quarters of social security earnings.
So, read the form before you sign, and be sure that you are willing to make a possible major commitment for the immigrant.