Cloud Top is a landmark located far north and a bit west of Chorrol. There is a trail that leads there. You must find Cloud Top as part of the Chorrol Recommendation Mage's Guild Quest. There isn't much to Cloud Top; a broken pillar (which looks vaguely special) surrounded by crumbling walls.
When you come to Cloud Top as part of the the Mage's Guild quest, there will be a charred body on the ground with the Fingers of the Mountain (book) (which you cannot read). After completing the Mage's Guild quest, you can return here with a Welkynd Stone and some kind of lightning spell to zap the pillar, causing you to learn the spell Finger of the Mountain.