In Scala, what exactly does 'val a: A = _' (underscore) mean?

Question

val a: A = _ is a compile error. For example:

scala> val a: String = _
<console>:1: error: unbound placeholder parameter
       val a: String = _
                       ^

What does work is var a: A = _ (note var instead of val). As Chuck says in his answer, this initialises the variable to a default value. From the Scala Language Specification:

0 if T is Int or one of its subrange types,
0L if T is Long,
0.0f if T is Float,
0.0d if T is Double,
false if T is Boolean,
() if T is Unit,
null for all other types T.

It initializes a to the default value of the type A. For example, the default value of an Int is 0 and the default value of a reference type is null.