This is the flag of Heard Island and McDonald Islands, the overseas territory of Australia, exactly the same as
🇦🇺 Australia.It is displayed as HM on some platforms. It generally means Heard Island and McDonald Islands or within Heard Island and McDonald Islands.
🇭🇲 is the Emoji of a country/region‘s flag, and its meaning is flag: Heard & McDonald Islands. The Emoji 🇭🇲 is composed of two regional indicator letters, namely 🇭 and 🇲. The country/region's 2-letter code for Heard & McDonald Islands is HM, so the letters corresponding to the two region indicator letters are H and M. 🇭🇲 is displayed as a whole flag on most platforms and as a two-letter symbol on a few platforms.