TL;DR: MMT starts by saying that governments that manage their own currencies have unlimited spending ability – they can simply create money when they need it. And that's obviously true, to the extent that people are willing to accept that money in exchange for their goods and services (an extent which is not infinite). Then, building on that premise, MMT basically claims that nobody needs to get hurt when governments create money out of thin air. The explanations for this are pretty wonky and hard to follow, but they amount to saying that the theory generally predicts no harmful inflation of the currency.

Analysis: Hasn't mankind already concluded that governments can't just create and spend money boundlessly without harming the economy and the people who constitute it? I'm looking at Argentina, Venezuela and Zimbabwe right now, where the governments have each created money to finance their spending, and people are truly suffering as a result. And those are just some contemporary examples – human history offers many more, and they're quite consistent. If MMT doesn't predict inflation as a result of arbitrary creation of currency, then it seems we can conclude MMT is incorrect.

Radical alternative theory: Governments should be fiscally responsible, practicing balanced budgets and promoting currency stability. Outlandish, true. But it turns out that historically this has been a very winning formula. More efficient economies, less corrupt governments, fewer innocent people getting hurt.