Splunk predict command period vs future_timespan?

I am wondering if anyone has an explanation of exactly what period is and what future_timespan is? I already read the document http://docs.splunk.com/Documentation/SplunkCloud/6.6.0/SearchReference/Predict which talks about both of the parameters but I am still really confused on what exactly they do and would like for someone to explain them to me in their own words. Thank you!