Shuffling means the reallocation of data between multiple Spark stages. “Shuffle Write” is the sum of all written serialized data on all executors before transmitting (normally at the end of a stage) and “Shuffle Read” means the sum of read serialized data on all executors at the beginning of a stage.
Your programm has only one stage, triggered by the “collect” operation. No shuffling is required, because you have only a bunch of consecutive map operations which are pipelined in one Stage.
Try to take a look at these slides:
http://de.slideshare.net/colorant/spark-shuffle-introduction
It could also help to read chapture 5 from the original paper:
http://people.csail.mit.edu/matei/papers/2012/nsdi_spark.pdf