3cbe9da3_pReduce___MongoDB_Manual_3_0_5__Description
[Field] out [Type] string or document [Description] 
Specifies where to output the result of the map-reduce operation. You can either output to a collection or return the result inline. On a primary member of a replica set you can output either to a collection or inline, but on a secondary, only inline output is possible. See out Options for more information.
mapReduce — MongoDB Manual 3.0.5
Description