It lacks document. A google search gives very few information. The only description I can find is in source code
/** Column update time delta on this Keyspace */
what's "time delta" mean? I often see a surge of this metrics when a new column family is added and read starts flowing in. https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/metrics/KeyspaceMetrics.java#L83
It gives the amount of time between updates to the same row in the memtables. This can give you a measurable estimate on the maximum clock skew before two consecutive updates may get applied in wrong order.