Spark Scala Understanding reduceByKey(_ + _)
<p>I can't understand reduceByKey(_ + _) in the first example of spark with scala  </p>

<pre><code>object WordCount {
def main(args: Array[String]): Unit = {
val inputPath = args(0)
val outputPath = args(1)
val sc = new SparkContext()
val lines = sc.textFile(inputPath)
val wordCounts = lines.flatMap {line =&gt; line.split(" ")}
.map(word =&gt; (word, 1))
.reduceByKey(_ + _)  **I cant't understand this line**
wordCounts.saveAsTextFile(outputPath)
}
}
</code></pre>