A wind barb is a compact way to represent both wind direction and wind speed in a single symbol. If there is wind, a staff will point to the direction the wind is coming from.
Wind speed is represented by the number of feathers and flags coming off the staff. If there is no wind, the barb is displayed as a circle with no staff or flags. A short feather represents 5 mph, a long feather is 10 mph and a flag (or pennant) is 50 mph. Summing the values of the feathers and flags gives the wind speed.