A time-series index file, also called an index file. A tsidx file associates each unique keyword in your data with location references to events, which are stored in a companion rawdata file. Together, the rawdata file and its related tsidx files make up the contents of an index bucket.
Each search you run scans tsidx files for the search keywords and uses their location references to retrieve from the rawdata file the events to which those keywords refer. To speed up searches, bloom filters narrow the set of tsidx files that Splunk Enterprise must search to get accurate results.
Splunk Enterprise creates a separate set of tsidx files for data model acceleration. In this case, it uses the tsidx files as summaries of the data returned by the data model.
For more information
In Managing Indexers and Clusters of Indexers:
In the Knowledge Manager Manual: