Bro, it's like this: Imagine you've got some shit, like stocks or whatever, and you gotta know what they're worth now, not what you paid for them. So, every day, you look at the current market price, and that's what your shit is worth.
Basically, MTM is: You're checking your "inventory" daily or whatever, and you slap the current market value on it. If the price goes up, you're like "fuck yeah, I'm rich!" If it drops, you're like "shit, I'm broke." It's all about what you could sell it for right now, not what you think it might be worth later.
Why it matters: It keeps you honest. No more pretending your Beanie Babies or whatever are still worth what you paid in '99. Financial institutions gotta do this to show real-time financial health, or else they risk being called out for holding onto worthless shit.
TL;DR: You value your assets or liabilities at today's price, so your financial statements don't look like you're still living in a 2007 housing bubble dream.
Meme version: It's like when you tell your mom you've been "investing" in video games, but she checks eBay and sees their current value is half what you paid, so you gotta explain why your "investment" strategy sucks.