降雨 is a formal, often meteorological verb-object compound (离合词) meaning 'rainfall' or 'to have rain/precipitation'. It's used in weather forecasts and scientific contexts; in everyday conversation 下雨 is more common for 'to rain'. As a separable verb, you can insert measure words or modifiers between 降 and 雨: 降了一场雨 ('a bout of rain'), 降了很大的雨 ('a heavy rain fell'). The object 雨 can be modified when separated.