官方文档的解释肯定是最准确的，要学会查找官方文档
docs.oracle.com

Determines whether to collect index statistics as part of gathering table statistics.

Specifying this option is equivalent to running the GATHER_INDEX_STATS procedure on each index of the table. Use the constant DBMS_STATS.AUTO_CASCADE to enable the database to determine whether index statistics need to be collected. This is the default. You can change the default using the SET_DATABASE_PREFS, SET_GLOBAL_PREFS, SET_SCHEMA_PREFS, and SET_TABLE_PREFS procedures.