It is a variety of hoe (raw dish) made with long arm octopus, a small octopus species called nakji in Korean and is sometimes translated into "baby octopus" due to its relatively small size compared to the giant octopus. The octopuses are most commonly killed before being cut into small pieces and served with the nerve activity in the octopus' tentacles still makes the pieces move posthumously on the plate whilst serving.
Because the suction cups on the arm pieces are still active when the dish is served, special care should be taken when eating sannakji. The active suction cups can cause swallowed pieces of the arm to stick to the mouth or throat. This can also present a choking hazard for some people, particularly if they are intoxicated. If you hold your life dear don't try eating moving octopus tentacles while drunk.