An attempt to seize power. I don't think he has anyone on his side except the military. But not in a despotic coup way, in a "declare martial law and they start out by following an apparently lawful order" way. Once the rest of the government responds I don't think the military will continue full martial law, just peacekeeping around parliament, and this guy will be removed.
Already officially voted down. Not sure about what's happening in practice though
Edit (from BBC):
The South Korean military says it will maintain martial law until it is lifted by President Yoon Suk Yeol, despite the nation's parliament voting to block its enforcement, according to the country's national broadcaster.