[improve][doc] Clarify the definition of message key
@@ -17,7 +17,7 @@ Messages are the basic "unit" of Pulsar. The following table lists the component
Component | Description
:---------|:-------
Value / data payload | The data carried by the message. All Pulsar messages contain raw bytes, although message data can also conform to data [schemas](schema-get-started.md).
-Key | Messages are optionally tagged with keys, which is useful for things like [topic compaction](concepts-topic-compaction.md).
+Key | The key (string type) of the message. It is a short name of message key or partition key. Messages are optionally tagged with keys, which is useful for features like [topic compaction](concepts-topic-compaction.md).
Properties | An optional key/value map of user-defined properties.
Producer name | The name of the producer who produces the message. If you do not specify a producer name, the default name is used.
Topic name | The name of the topic that the message is published to.
