What does this error mean?

This error means that the index summary resizing process was interrupted and could not continue.

Why does this error occur?

Cassandra manages the memory used by the index summaries by periodically resizing them in order to give more memory to hot sstables and less memory to cold sstables. This process is performed as a compaction operation. If the operation is stopped (e.g. nodetool stop INDEX_SUMMARY) the error message logged will be

Got exception during index summary redistribution. Compaction interrupted: <compaction info>.

How do you fix this error?

There is nothing to fix in this case unless you can verify that there was no operational cause for the stop to occur. If a manual intervention cause the error to be thrown, it can be safely ignored as Cassandra will again attempt an index summary resize. If the error occurs with no operator input, check the system.log of Cassandra for any additional messages that will add details to why the process was stopped.  The OS logs may provide information as well. If the information you find is vague, or you cannot find any information, please open a ticket with DataStax Support.