json_populate_recordset

From here:

https://www.postgresql.org/docs/10/static/functions-json.html

select * from json_populate_record(null::myrowtype, '{"a": 1, "b": ["2",
"a b"], "c": {"d": 4, "e": "a b c"}}')

What is the null::myrowtype doing?

Thanks,
--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com