Umbrellas can be purchased from most well-stocked city stalls or haberdashers about the land. They are useful for keeping you dry during rainstorms (providing the storm is not too strong/windy) and this has many benefits, since it's likely your inventory will hold numerous items that won't survive well if drenched by a sudden downpour.
To use an umbrella you must wield it in one of your hands - convention says left hand, so WIELD LEFT UMBRELLA to hold it in place.
Then UNLATCH UMBRELLA will open it, and LATCH UMBRELLA will close it again. When it is open it will shield you as best it can but will be vulnerable against high winds or torrential cloudbursts. When it is closed it will not provide you with any shielding against the elements but, if you deem the conditions dangerous for the umbrella, you may wish to close it and make a beeline for more sturdy permanent cover rather than risk it being broken by a sudden galeforce gust.