SAREB is an acronym in Spanish for Sociedad de Gestión de Activos procedentes de la Reestructuración Bancaria. It is a holding bank (a bit like NAMA in Ireland), established in 2012 in Spain that holds property and other assets transferred to it by four nationalised banks to get the bad assets off their balance sheets. The four banks are BFA-Bankia, Catalunya Banc, NGC Banco-Banco Gallego and Banco de Valencia. It is supposed to sell off these assets over a 15 year period.
You can read more about it here