An index provides a means to access data in Cassandra using attributes other than the partition key. The benefit is fast, efficient lookup of data matching a given condition. The index indexes column values in a separate, hidden table from the one that contains the values being indexed. Cassandra has a number of techniques for guarding against the undesirable scenario where a data might be incorrectly retrieved during a query involving indexes on the basis of stale values in the index.
As mentioned earlier, in Cassandra 2.1 and later, you can index collection columns.