How good for a horse is 5 out of 10 on the temperament scale?

I'm assuming you're talking about online horse advertisements (like the ones found on equine.com, dreamhorse.com, and similar sites)

5 would be in the middle of the scale of 1-10, meaning that the horse is about in the middle of extremely calm and extremely excitable. This would be a horse for an intermediate to experienced rider willing to spend time working with a more difficult horse.