This means that you can use the index field to filter your search results by the name of the index that contains the events you want to see.
For example, if you want to search for events in the index named "gcp_logs", you can use the following SPL:
index=gcp_logs
You can also specify multiple indexes by using the OR operator, such as:
index=gcp_logs OR index=oswin

Explanation/Reference: https://docs.splunk.com/Documentation/Splunk/7.2.6/Search/Aboutsubsearches

Explanation/Reference: https://docs.splunk.com/Documentation/Splunk/7.2.6/Search/Usethetimeline