A lightning bolt in a dream or a flash of lightning, or an experience with lightning in a dream is a wakeup call with a warning. Just like it is that something is about to come down in your waking world, so too is the symbol in your dream world. The Tower Card in the Tarot most closely resembles the significance of lightning in your dream, and even depicts the symbol of lightning on the card in some decks.
This card, and this symbol in your dream, is a warning of sorts that a storm is brewing. Like the Tower, the storm is an environmental concern, meaning it is not something that you can control in terms of whether or not it is going to happen. This is an external event that will cause some problems for a little while, but, like every storm, will eventually pass.