Splunk ML - forecast time series

Hi there, I started using splunk machine learning but trying to understand on how to use forecast time series.Can someone please explain what is the holdback , future timespan. When i read the documentation and try to set the values is throwing an error "External search command 'predict' returned error code 1."I am also trying to understand all the methods like LLP5,LL,LLP,LLT. It would be great, if someone could be help to understand the forecast time series in ML.