http://travel.state.gov/content/visas/english/immigrate/immigrant-process/documents/support.html
Establish Financial Support
About the Affidavit of Support
The Affidavit of Support (AOS) is a legal contract required by U.S. law between the petitioner (sponsor) of an immigrant visa applicant and the U.S. government. It ensures that the visa applicant has adequate means of financial support and is unlikely to become a
public charge after entering the United States. The sponsor for the prospective immigrant must establish he or she has the means to financially support the intending immigrant(s), if necessary. For full information about Affidavit of Support requirements, forms, and whether an Affidavit of Support is required for you, review
Affidavit of Support on the U.S. Citizenship and Immigration Services website.