mapreduce_shuffle in this config file is part of Plugable Shuffle and Sort. Shuffle and Sort are what connect the mappers to the reducers. A nice graphical representation of this is the following (shuffle is called "copy" in this figure):
The Hadoop arcitecture alllows these steps to be customized:
The pluggable shuffle and pluggable sort capabilities allow replacing the built in shuffle and sort logic with alternate implementations. Example use cases for this are: using a different application protocol other than HTTP such as RDMA for shuffling data from the Map nodes to the Reducer nodes; or replacing the sort logic with custom algorithms that enable Hash aggregation and Limit-N query.
mapreduce_shuffle parameter is mentioned in the Hadoop Docs, and is the default value for this configuration.