A special index that stores the results of a scheduled report, when you enable summary indexing for the report. Summary indexing lets you run fast searches over large data sets by spreading out the cost of a computationally expensive report over time. To achieve this, the search that populates the summary index runs on a frequent, recurring basis and extracts the specific data that you require. You can then run fast and efficient searches against this small subset of data in the summary index.
In the Knowledge Manager Manual: