The fill_summary_index.py script is a utility provided by Splunk to backfill data into a summary index. It's particularly useful when there are gaps in the summary index due to missed scheduled searches or when initializing a summary index with historical data.
According to Splunk Documentation:
"You can use the fill_summary_index.py script, which backfills gaps in summary index collection by running the saved searches that populate the summary index as they would have been executed at their regularly scheduled times for a given time range." Reference:Manage summary index gaps - Splunk Documentation