What is `String("a")` type

I tried the following code below:

scala> // Scala 2.13.8

scala> val a = "a"
val a: String = a

scala> val a: "a" = "a"
val a: "a" = a

scala> final val a = "a"
val a: String("a") = a

I know that String is normal type and "a" is a literal type.

What is String("a")?