Fiber represents the (pure) result of a IO being started concurrently and that can be either joined or cancelled.

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 task = UIO.evalAsync(println("Hello!"))

val forked: UIO[Fiber[Nothing, Unit]] = task.start

Usage example:

val launchMissiles = Task(println("Missiles launched!"))
val runToBunker = Task(println("Run Lola run!"))

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