1. USDT > USD. This is the benign version. This signifies increased demand for Tether itself. This happens when there are arbitrage opportunities between exchanges that can only be exploited quickly using Tether. This is the variety that is most common, and exactly what Tether was intended to be used for.
2. USDT < USD. This is bad. This means that people are willing to take a small haircut to not be holding Tether. This means that the market views Tether as risky in some way, possibly implying insolvency. This has happened a few times, but is less common.
Understanding this is essential to interpreting Tether, price divergences, and also the nominal divergence between prices on different exchanges.