The dream or vision of thunder tells you about the reflection of inner anger, aggression, fear and emotions that want to express you in a powerful and persistent way. Another explanation is the reference to negative character traits that you let go.
Another explanation is that new insights will come unexpectedly, revelations, in your life, suddenly you see the usefulness of a situation in your life. An alternative explanation are disappointing results and loss, something happens in your life that leaves you with a sad feeling. Yet another explanation is that thunder represents the connection to the universe, the afterlife, gods and goddesses with which you feel connected.