Fiber represents the (pure) result of a Concurrent data type (e.g. IO) being started concurrently and that can be either joined or canceled.

You can think of fibers as being lightweight threads, a fiber being a concurrency primitive for doing cooperative multi-tasking.

For example a Fiber value is the result of evaluating IO.start:

val io = IO.shift *> IO(println("Hello!"))

val fiber: IO[Fiber[IO, Unit]] = io.start

Usage example:

for {
  fiber <- IO.shift *> launchMissiles.start
  _ <- runToBunker.handleErrorWith { error =>
    // Retreat failed, cancel launch (maybe we should
    // have retreated to our bunker before the launch?)
    fiber.cancel *> IO.raiseError(error)
  }
  aftermath <- fiber.join
} yield {
  aftermath
}