The mainstream way of thinking says "the government must tax in order to spend" whereas MMT says "if the government wants to spend it will have to tax".
I think it's more "the government must spend to be able to tax"?
My understanding of MMT is that it removes the direct relationship between the ideas of spending and taxation, instead tying them both to inflation. Spending increases inflation by adding currency into circulation, and taxation decreases inflation by removing it from circulation. Although I'm not sure how that fits into a one-liner :)