Usch is used as an interjection in Swedish, meaning something like 'yuck', 'ugh' or 'ew'.
It often expresses disgust, distaste or dislike, for example if you're eating or talking about an unappetising food, miserable weather, or something that's repulsive in some other way.
But it doesn't always have that connotation, and is sometimes used to convey other strong emotions. In sentences like 'usch, någonting är fel' (ugh, something is wrong), 'usch, vad pinsamt' (ugh, how embarrassing) or 'usch, jag är nervös' (Oh, I'm nervous) it shows discomfort.
And you might say something like 'usch, vad soligt!' (wow, it's sunny!) even if sunshine is usually a positive, in situations where the sun is causing you discomfort – for example if you forgot to bring your sunglasses or suncream out with you.
You could also use it to fill an uncomfortable silence: imagine there's just been an argument and someone's stormed off. While everyone left is wondering what just happened and what to do next, someone might say 'Usch'.
Usch, vilket väder!
Ugh! What miserable weather!
Usch, vad är det som luktar?
Yuck, what's that smell?
Usch, nu måste vi prata med henne
Yikes, now we have to talk to her