When Jonas receives the memories from the Giver, his consciousness is sort of mixed. His consciousness is mixed between his real life and the memory that he the Giver is giving to him.
So, in the first time when he is given a memory, Jonas is aware that he is still lying in the room. But at the same time he is really living what happens in the memory. Similarly, he thinks words that mean something to him in real life (mound) but at the same time he feels words that he didn't know before (hill).