Descartes Meditations 1: What role do dreams play in “destroying” his former beliefs?

In the Meditations Descartes is trying to establish absolutely certain grounds for our knowledge. For many thinkers, such certainty can be provided by our sensory experience of the world around us. This is a school of philosophy called empiricism. Descartes, however, is a rationalist. Rationalism argues that the ultimate nature of things can only be established by reason. On this account, the senses are inherently unreliable; they cannot provide us with certain knowledge. That stick that appears bent in the water really isn't; the moon is much bigger than it appears in the sky; and so forth.
For Descartes, dreams represent an extreme example of how sensory experience can lead us astray. They destroy any empirical certainty we may have about the world. The senses can't determine for sure whether or not we're dreaming; only the reasoning faculty can do this. If we simply relied on our senses, we'd never be certain that what we are experiencing at any one time is real, as opposed to just a dream. The senses play tricks on us, whether we're awake or asleep. There are various levels of trickery, some more obvious than others, but it's only by the exercising of the reasoning faculty that we're able to know this. The senses can't help us in this regard; for Descartes they are part of the problem, not the solution.