system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Trillion dollar coin

Wikipedia wasn't very helpful and Googling didn't yield much in the way of 'why?'
gpt
The current answers here completely misunderstand the purpose of the coin.

The government (and most governments worldwide) spend more money than they take in.  To cover the difference, they issue debt (also called government bonds, T bills, treasuries, etc.).  This is similar to putting some of your monthly expenses on a credit card.  The difference is that this isn't really as bad for a government as it is for a person for some various reasons that are beyond the scope of your question.

Now, for some political reasons, in the US, the Congress has set a legal limit on the amount of debt that can be issued (the debt ceiling).  The debt ceiling doesn't go up with inflation or when Congress tells the government to spend more money than it takes it (new programs or population growth that makes current programs more expensive), so we keep running up against the debt ceiling.  Legally, the government (the Secretary of the Treasury) can't issue any more debt, so the US would basically run out of money in its bank account, and Congress has said that it's not allowed to use its credit card anymore.

Normally money is issued by the Federal Reserve (the Fed), which is an independent body that the president and the Secretary of the Treasury don't have direct control over.  This is setup this way so that the president can't issue a ton of new money and cause inflation.  The Fed is our promise to people that we'll keep inflation at ~3% or so rather than just printing a bunch of dollars.

The coin is a loophole.  The only money that the Secretary of the Treasury is allowed to "print" are platinum coins.  All other money is printed by the Fed.  Why platinum coins?  The goal was to let the government print a few commemorative coins to sell to coin collectors and make a few bucks but not really influence inflation or the money supply.   *But*, there's no reason why the government can't just print a trillion dollar platinum coin (the loophole).

Why would this not cause massive inflation?  It's because basically this coin would act very similarly to issuing more debt.  The government prints this coin and deposits it in its bank account (the Fed).  Voila, the government can use this to pay down its credit card balance and now has access to that extra amount of credit.  But in this case, the government would promise to later destroy this money as soon as Congress gets its act together and raises the debt ceiling.  In other words, while it is legally money, the government would promise that it's actually more like an IOU.  So in that sense, it's like if you hit your credit limit and have no money in your bank account, you borrow money from your brother *and* the credit card company has already told you that it will raise your credit limit, it's just working through some paperwork.  As soon as your "official" credit line from the credit card company is raised, you can pay back the "unofficial" money that you took from your brother.

Because of the current economy, US government debt effectively pays investors a 0% interest rate (investors are so scared and they view US debt as so safe, that they're basically parking their money with us for no charge).  In the earlier example, if your brother didn't need that money, then it wouldn't really matter that he loaned it to you for free.  On the other hand, if his bank was willing to pay him a lot of interest, he might be irritated because he won't make that interest money while you borrowed from him.

tl;dr Because the government would promise to destroy the coin after the debt ceiling was raised, the coin actually acts as a debt instrument.