Understanding Scalaz Kleisli

Question

I often write code like this:

abstract class Foo [T[+_]: Monad]
{
  def getStr (): T[String]
}

object Example
{
  def f [T[+_]: Monad] (bar: String): ReaderT[T, Foo[T], String] = {
    Kleisli { foo =>
      for {
        str <- foo.getStr ()
      } yield str + bar
    }
  }
}

I know what the code above does, however I do not fully understand the inner workings of the Kleisli part.

Looking at the source I can see that the Kleisli type represents a function that takes type and returns another type wrapped in a higher order type:

/**
 * Represents a function `A => M[B]`.
 */

How can that be used to explain the example I've posted above?

What else can Kleisli be used for?