MMT is the idea that the constraint on tax/spending in a fiat money system is monetary effects (inflation/deflation), not the need to tax or borrow money that you could just print to fuel spending.

Ironically, the people that are most opposed to MMT on the basis of outright lies about its content are the people that are most likely to argue against large deficits on inflation grounds even when there is no problem borrowing enough money to fuel it.