This stands for National Security Entry-Exit Registration System. It is a system for registering certain non-citizens within the United States, initiated in September 2002 as part of the War on Terrorism. Portions were suspended as of April 27, 2011.
This system has two separate components: port-of-entry registration and domestic registration. In each case, those who register are fingerprinted, photographed, and interrogated. They are required to provide detailed information about their plans and to update Immigration and Customs Enforcement (ICE) if their plans change. They are only permitted to enter and depart the United States through designated ports of entry.