What are the actual implications of MAD? [closed]

Many people are against western intervention in Ukraine due to MAD (mutually assured destruction). But what does MAD actually mean? Is it just a war that would kill a lot of people? Or is it genuinely, actually, total destruction of entire nations, and possibly the world?

I feel like I have no clear understanding of this, and it seems to me quite crucial to understand. Take for example a country like the USA. If MAD means "a lot of people will die, but USA as a nation will probably survive, especially the rich who can bunker down", then that makes a nuclear war between Russia and USA waaaay more likely (to me, at least) than if MAD means "everyone is going to die, society is going to collapse, the rich and powerful are going to lose all that money and all that power".

So which one is it?