You're understanding tariffs correctly, but the conclusion is off. I'll give an example driving tariffs to the extreme to explain:

Imagine tariffs are so high, the US can't economically import anymore. This is effectively the same as very tight sanctions, aka like North Korea.

Everything in North Korea is manufactured locally, and it doesn't scale. Doing things locally works great, but you focus on a couple things and trade for the rest.

Can some tariffs help the economy? Definitely, it's a bit of a catalyst. Will a lot of tariffs hurt the economy? Also definitely.

Maybe Trump has mathed out the exact level of tariffs to boost the economy. If, however, that is the case, he hasn't shown his work on it.

Edit: I will caveat it is clear most people in this thread saying "even I know what a tariff is!" Are mostly wrong. So if that's why you're feeling crazy, it's because you're in a thread full of people who aren't as smart as they think they are.