Explain the Reducer’s reduce phase?

In this phase the reduce(MapOutKeyType, Iterable, Context) method is called for each pair in the grouped inputs. The output of the reduce task is typically written to the
FileSystem via Context.write(ReduceOutKeyType, ReduceOutValType). Applications can use the Context to report progress, set application-level status messages and update
Counters, or just indicate that they are alive. The output of the Reducer is not sorted.