What does Jonas dream about in Chapter 10?
Chapter 10 is where Jonas first meets the Giver and begins his training as the Receiver of Memories. He does not dream in this chapter, however, the dream you may be referring to is the sexual dream Jonas has about Fiona in Chapter 5. He dreams that he is trying to convince Fiona to get undressed and get in the tub at the house of the old so he could bathe her. When he reports this dream to his family, as everyone in the community must do after they dream, his parents tell him that he had "the Stirrings" and they give him a pill so that he would no longer have any sexual feelings. No one in the community has sexual relations, procreation is controlled and there are assigned women who have the role of "Birthmother."
The other dream Jonas has is in Chapter 12, where he replays the memory of sledding in the snow. This dream is a foreshadowing of the end of the book -- he dreams he is heading towards a destination in the snow, and he needs to reach that place, that it is a significant place.
In Chapter 10 when Jonas goes to begin his training, The Giver tries to explain his mental state by comparing it to sledding down a snowy hill. Jonas didn't know what a sled was nor snow. The Giver says that he will give Jonas the memory of snow. This is when Jonas "dreams" again. This is explained on into Chapter 12 when he tells his mother that he hadn't had any dreams, but he had in fact dreamed of sledding in the snow. I believe that this is the dream you are referring to.