If you see this error message in your query log, it means that the running query has been cancelled. This can happen for one of two reasons:
1. The query was manually cancelled, either from the Data Explorer or the Query Log.
2. The query exceeded the Maximum Query Duration setting for your data source.
Generally, it's a good idea to keep the Maximum Query Duration set to 5 minutes or less. Any queries that take longer than 5 minutes should be optimized, because they tie up database resources and can prevent other charts from loading.
For general tips on optimizing query performance. see our Knowledge Base article.