This is a stand-by signal, indicating that a tropical cyclone is centred within about 800 km of Hong Kong and may affect the territory.
Action - If you are planning an outing, remember that there is a tropical cyclone near Hong Kong which may affect your plans. Beware that strong winds may occur over offshore waters. Listen to radio and TV broadcasts or browse the Observatory website on the progress of the tropical cyclone.
Now for translation, there's a tropical storm about 390 miles south to southwest of us. Sounds like Savannah, right? You can check out the details on Wunderground. For now, we'll go for a swim