REF2015 is the 2015 version of the Rosetta Energy Function. REF2015 is used to calculate the "free energy" of a protein, which is converted to the score in Foldit.
REF2015, is described in detail in The Rosetta All-Atom Energy Function for Macromolecular Modeling and Design (HTML). Various terms or values in REF2015 are grouped to give Foldit score parts and the resulting subscores.
REF2015 is the version of Rosetta currently used by Foldit.