What does a lazy val do?

I noticed that Scala provide lazy vals. But I don’t get what they do.

scala> val x = 15
x: Int = 15

scala> lazy val y = 13
y: Int = <lazy>

scala> x
res0: Int = 15

scala> y
res1: Int = 13

The REPL shows that y is a lazy val, but how is it different from a normal val?