Shuffling and Sorting are two major processes operating simultaneously during the working of mapper and reducer.
The process of transferring data from Mapper to reducer is Shuffling. It is a mandatory operation for reducers to proceed their jobs further as the shuffling process serves as input for the reduce tasks.
In MapReduce, the output key-value pairs between the map and reduce phases (after the mapper) are automatically sorted before moving to the Reducer. This feature is helpful in programs where you need sorting at some stages. It also saves the programmer's overall time.
Learn all about shuffling and sorting in this comprehensive MapReduce Tutorial.