What does ':_*' (colon underscore star) do in Scala

colon_underscore_star.scala

  // :*_ tells the compiler to pass each element of arr as its own argument to echo, rather than all of it as a single argument.
  // It is useful when converting list to Map in Scala
  var mutableMapInstanceOfItemToBoolean = scala.collection.mutable.Map(arrayOfSomeSort.values.toList.map(item => item -> false):_*)