Override Return Timeframe means the timeframe in which a customer can return a product for certain reasons also specified by you in the Returns Policy. 

You can set your own default time limit for product returns, be it 30 days or less, in the Returns Policy on your AppScenic Account, under Products. 

By checking the Override Return Timeframe box on a specific product page, you disable the default timeframe and set a new one that only applies only to that product. 

So now you know what it means when you check Override Return Timeframe.