toDF是一个扩展方法。通过导入，您可以将必要的隐式带入作用域。
例如，Int没有方法foo

1.foo() // doesn't compile

但如果定义扩展方法并隐式导入

object implicits {
  implicit class IntOps(i: Int) {
    def foo() = println("foo")
  }
}

import implicits._
1.foo() // compiles

编译器将1.foo()转换为new IntOps(1).foo()。
同样地

object implicits extends SQLImplicits ...

abstract class SQLImplicits ... {
  ...

  implicit def rddToDatasetHolder[T : Encoder](rdd: RDD[T]): DatasetHolder[T] = {
    DatasetHolder(_sqlContext.createDataset(rdd))
  }

  implicit def localSeqToDatasetHolder[T : Encoder](s: Seq[T]): DatasetHolder[T] = {
    DatasetHolder(_sqlContext.createDataset(s))
  }
}

case class DatasetHolder[T] private[sql](private val ds: Dataset[T]) {

  def toDS(): Dataset[T] = ds

  def toDF(): DataFrame = ds.toDF()

  def toDF(colNames: String*): DataFrame = ds.toDF(colNames : _*)
}

import spark.sqlContext.implicits._将spark.sparkContext.parallelize(List(1,2,3,4,5,6,7,8,9)).toDF转换为rddToDatasetHolder(spark.sparkContext.parallelize...).toDF，即DatasetHolder(_sqlContext.createDataset(spark.sparkContext.parallelize...)).toDF。
你可以在Scala中读到隐式和扩展方法
Understanding implicit in Scala
Where does Scala look for implicits?
Understand Scala Implicit classes
https://docs.scala-lang.org/overviews/core/implicit-classes.html
https://docs.scala-lang.org/scala3/book/ca-extension-methods.html
https://docs.scala-lang.org/scala3/reference/contextual/extension-methods.html
How extend a class is diff from implicit class?
关于spark.implicits._
Importing spark.implicits._ in scala
What is imported with spark.implicits._?
import implicit conversions without instance of SparkSession
Workaround for importing spark implicits everywhere
Why is spark.implicits._ is embedded just before converting any rdd to ds and not as regular imports?