what is mapreduce in pyspark code example

Example: pyspark mapreduce dataframe

df.rdd \
  .filter(lambda x: x[1] == "france") \ # only french stations
  .map(lambda x: (x[0], x[2])) \ # select station & temp
  .mapValues(lambda x: (x, 1)) \ # generate count
  .reduceByKey(lambda x, y: (x[0]+y[0], x[1]+y[1])) \ # calculate sum & count
  .mapValues(lambda x: x[0]/x[1]) \ # calculate average
  .sortBy(lambda x: x[1], ascending = False) \ # sort
  .take(100)