PRECIP_AVG consists of a set of 12 separate variables. These variables report the average precipitation in millimeters, received within a 10 kilometer circular buffer around each DHS cluster location, during each calendar month (January through December), averaged over the 1981-2016 timeframe.
By adding PRECIP_AVG to your data cart, you are in effect adding 12 separate variables (PRECIP_JAN - PRECIP_DEC), one variable for each month.
Please see the User Note on Using IPUMS-DHS Contextual Variables: Precipitation [URL omitted from DDI.] for the data source, methodology, and more detailed information about these variables.