An index is a structure maintained by a persistent class that is intended to be used to optimize queries and other functions. These indices are automatically maintained whenever INSERT, UPDATE, or DELETE SQL-based operations are carried out against the database; likewise for object-based actions. The SQL Query Processor makes use of available indices when preparing and executing SQL queries. You can add index definitions to persistent classes. They are not meaningful in other kinds of classes.
See Index Syntax and Keywords in this reference.