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.
Only because lawmakers literally broke into parliament to have the vote since the military was complicit with Yoon into not letting them in.
If 190 didn't have the balls to break into parliament, there would have been a quick downward spiral of military lead authoritarianism.
The military was in on this. All other lawmakers said they were not aware of this and the tanks rolled in minutes after the order which means they were prepared for the announcement in secret. ...and he will try again... My guess is when he is impeached for this coup attempt.
However next time they won't let them in the building as easily since that was the key to stopping it.