"The Cloud," by Percy Bysshe Shelley, is a poem that heavily relies on a series of literary devices, such as personification. The poem is actually told from the point of view of the cloud itself, which is presented as a being of almost mythical nature. The cloud is the element that is needed to connect and support every other aspect of nature. Many other elements of nature are also personified.

We encounter the leaves you are referring to in the first stanza, which describes some of the cloud's activities. The cloud provides water for thirsty flowers and wakes sleeping buds with morning dew. The cloud is also the source of shade for the leaves as they sleep during the day. Therefore the answer to your question is that the cloud provides shade for the leaves while they dream. As we can see, flowers, leaves, and also many other elements of nature we encounter later on in the poem are all personified. 

I bear light shade for the leaves when laid In their noonday dreams.