This is somewhat fairly straightforward - it's a smoke mound. Used for sending smoke signals. There are 38 such smoke mounts on Jeju Island, which were used to send warning signals when enemy ships were approaching in order to prepare defensive positions.

As you may know from other cultures that used smoke signals, it's a great way to send distress signals over long distances quickly, especially if there are multiple relays. This works during the day or evening in fair weather, however if it was cloudy or rainy, the mound keeper would have to run in order to spread the news.

Smoke mound on Jeju Island, South Korea.