Scala - Operation in case (x,y)=> x++y

I am new to scala. I was reading one code not able to understand . Can someone please help me to understand below code ?

 def intersectByKey[K: ClassTag, V: ClassTag](rdd1: RDD[(K, V)], rdd2: RDD[(K, V)]): RDD[(K, V)] = {
    rdd1.cogroup(rdd2).flatMapValues{
      case (Nil, _) => None
      case (_, Nil) => None
      case (x, y) => x++y
    }
  }

What does below line means ? How it will be evaluated ?

case (x, y) => x++y

Thanks