Hm, imagine your ship made really big waves when swimming. Below the speed of waves you don’t really need to worry about them because you don’t really experience them - they just run ahead of you.

If your speed is close to the speed of waves, the waves that you produce get stuck on top of one another, so you have this huge wave ahead of you.

If you want to break through that barrier, you need a really strong ship to plow through that huge wave, but once you’re through it, it gets smoother - you don’t encounter your own waves any more.

But now the problem is: the moment you slow down, or make a turn, all the waves you produced will hit you from the back.

We don’t get that with water due to various reasons, but we get that with air and sound.

In simplistic terms, at supersonic speeds the air particles do not have sufficient time to "get out of the way" of the body moving in the air. This causes the air particles to be compressed into a dense "shock wave", which can cause separation of the air flow from the surface of the airfoil, leading to a stall. The properties of a wing that handles supersonic airflow well are often at odds with a subsonic wing.

The speed of sound is vaguely the speed at which air can get out of the way. If you try travelling faster than that the air bunches up into a shockwave, which causes all kinds of problems and is generally different to travelling through air slowly.