This was mentioned in the Kotlin FAQ http://kotlinlang.org/docs/reference/faq.html
The idea was to support special string literals written in another language (e.g. SQL) with injected parts of Kotlin. Something similar to Language Injection feature of IntelliJ IDEA, but supported by compiler.
This FAQ was written long time ago, when implementation of Kotlin had only started. This feature is unlikely to be implemented in Kotlin 1.0, so we’d better remove it from FAQ.