Hats are usually symbolic of power and authority.
They also stereotype the person that is wearing it.
Look at the details in your dream especially who is wearing the hat.
The type and quality of the hat usually represents the degree of authority and respect that your unconscious mind is giving to the person wearing it.
Generally the person wearing the hat is representing a part of you.
What is the hat saying about your position in life and your attitude toward it?
This is a variable dream and depends more on the action in the dream rather then the hat itself.
These are a few meanings: A new elaborate hat denotes a rise in status and good luck, while old shabby hats are a wide range of bad luck; if you lose your hat you will have a loss of finances, etc..