Scala's lazy arguments: How do they work?

In the file Parsers.scala (Scala 2.9.1) from the parser combinators library I seem to have come across a lesser known Scala feature called "lazy arguments". Here's an example:

def ~ [U](q: => Parser[U]): Parser[~[T, U]] = { lazy val p = q // lazy argument
  (for(a <- this; b <- p) yield new ~(a,b)).named("~")
}

Apparently, there's something going on here with the assignment of the call-by-name argument q to the lazy val p.

So far I have not been able to work out what this does and why it's useful. Can anyone help?