Each mapping can have custom meta data associated with it. These are simple storage elements that are simply persisted along with the mapping and can be retrieved when fetching the mapping definition. The meta is defined under the _meta element, for example:

{
    "tweet" : {
        "_meta" : {
            "attr1" : "value1",
            "attr2" : {
                "attr3" : "value3"
            }
        }
    }
}

Meta can be handy for example for client libraries that perform serialization and deserialization to store its meta model (for example, the class the document maps to).