People throw around terms so often that this is actually an excellent question! Basically, the background behind so-called stable coins, which include Tether above all, is the minimisation of massive price fluctuations. Hence the word stable as in firm, constant, or unwavering. Which you may know, is about the exact opposite of the experience with Bitcoin, whose price can be all over the place, even within a given day.
In order to achieve this steadiness, stable coins are linked to fixed values. For example, in the case of USDT, to the exchange rate of the US dollar. This means it will only fluctuate as much as the value of the dollar. And while it has seen a little more movement than usual these days, again, it is no comparison to the wild fluctuations of things like Bitcoin. Comparatively, you could look at it as the difference between investing in government bonds versus the high-tech sector. One is solid with little movement and predictable results. The other is volatile but has far greater profit potential.
Strictly speaking, there are three different forms of stable coins. They can be linked to the base value of a real money currency like USDT is, they can be linked to other cryptocurrencies, or they can be linked to certain algorithms.