diff --git a/src/Interpreters/ProcessList.h b/src/Interpreters/ProcessList.h
index 799309541ee..acfa96326c7 100644
--- a/src/Interpreters/ProcessList.h
+++ b/src/Interpreters/ProcessList.h
@@ -48,6 +48,9 @@ enum CancelReason
     UNDEFINED,
     TIMEOUT,
     CANCELLED_BY_USER,
+
+    /// CANCELLED_BY_ERROR means that there were parallel processes/threads and some of them failed,
+    /// so we cancel other processes/threads with this cancel reason.
     CANCELLED_BY_ERROR,
 };