They are memories but the random nature of dreaming means that it is impossible to extract any meaningful or accurate information from them.
Dreaming is essentially the process of memory consolidation. While your body is asleep, your memories stored in the hippocampus become semantic memories, which are stored in the temporal lobe. As the hippocampus sends experiences and memories to the temporal lobe, it interacts with other parts of the brain. These seemingly random interactions with semi-conscious parts of the brain cause us to have dreams.
This explanation for the functionality and purpose of dreaming is rooted in science. A 2007 study found that even when mice are asleep, the neocortex continues to remain active and communicates with the hippocampus. The neocortex then decides what memories must be preserved and stored as long-term memories and what must be discarded.
The random nature of these interactions means that even though our dreams are affected by our waking experiences and our memories, they cannot tell us anything informative. Trying to interpret someone's past from their dreams would be like trying to understand mathematics by looking at an endless sequence of random numbers. Just because memories are the building-blocks of dreams, as numbers are for mathematics, does not mean they are interpretable.
As a result, no dream interpretation can contain any accuracy.