Scala's underscore wild card used as placeholder for anonymous functions

placeholder.scala

val songTitles = List("Lay My Soul to Waste", "Shallow Grave", "Killer by Night", "Cold Dark Mourning")
                                                   
//passing anonymous function (String => String) to map
songTitles.map(t => t.toLowerCase)                                                              
//same using placeholder syntax:
songTitles.map(_.toUpperCase)