Map reduce is very prevalent in distributed data processing.
It's an idea from functional programming. The basic idea is you have a map function that can apply a filter to a list, and then apply a summary operation, which is the reduce.
A Mickey Mouse example to bring this to life could be - you have list of fishing vessels, you want to filter for foreign flags, and then you want to add them all up, and you want to do this every 6 hours and create a time series database of this data.
A popular open source implementation is Apache Hadoop.